publish_subcommand hook + default plugins mechanism, used for publish heroku/now (#349)

This change introduces a new plugin hook, publish_subcommand, which can be
used to implement new subcommands for the "datasette publish" command family.

I've used this new hook to refactor out the "publish now" and "publish heroku"
implementations into separate modules. I've also added unit tests for these
two publishers, mocking the subprocess.call and subprocess.check_output
functions.

As part of this, I introduced a mechanism for loading default plugins. These
are defined in the new "default_plugins" list inside datasette/app.py

Closes #217 (Plugin support for datasette publish)
Closes #348 (Unit tests for "datasette publish")
Refs #14, #59, #102, #103, #146, #236, #347

docs/datasette-publish-heroku-help.txt

@@ -0,0 +1,20 @@
+$ datasette publish heroku --help
+
+Usage: datasette publish heroku [OPTIONS] [FILES]...
+
+Options:
+  -m, --metadata FILENAME   Path to JSON file containing metadata to publish
+  --extra-options TEXT      Extra options to pass to datasette serve
+  --branch TEXT             Install datasette from a GitHub branch e.g. master
+  --template-dir DIRECTORY  Path to directory containing custom templates
+  --plugins-dir DIRECTORY   Path to directory containing custom plugins
+  --static STATIC MOUNT     mountpoint:path-to-directory for serving static files
+  --install TEXT            Additional packages (e.g. plugins) to install
+  --version-note TEXT       Additional note to show on /-/versions
+  --title TEXT              Title for metadata
+  --license TEXT            License label for metadata
+  --license_url TEXT        License URL for metadata
+  --source TEXT             Source label for metadata
+  --source_url TEXT         Source URL for metadata
+  -n, --name TEXT           Application name to use when deploying
+  --help                    Show this message and exit.

docs/datasette-publish-now-help.txt

@@ -1,31 +1,23 @@
-$ datasette publish --help
+$ datasette publish now --help
 
-Usage: datasette publish [OPTIONS] PUBLISHER [FILES]...
-
-  Publish specified SQLite database files to the internet along with a datasette API.
-
-  Options for PUBLISHER:     * 'now' - You must have Zeit Now installed:
-  https://zeit.co/now     * 'heroku' - You must have Heroku installed:
-  https://cli.heroku.com/
-
-  Example usage: datasette publish now my-database.db
+Usage: datasette publish now [OPTIONS] [FILES]...
 
 Options:
-  -n, --name TEXT           Application name to use when deploying
   -m, --metadata FILENAME   Path to JSON file containing metadata to publish
   --extra-options TEXT      Extra options to pass to datasette serve
-  --force                   Pass --force option to now
   --branch TEXT             Install datasette from a GitHub branch e.g. master
-  --token TEXT              Auth token to use for deploy (Now only)
   --template-dir DIRECTORY  Path to directory containing custom templates
   --plugins-dir DIRECTORY   Path to directory containing custom plugins
   --static STATIC MOUNT     mountpoint:path-to-directory for serving static files
   --install TEXT            Additional packages (e.g. plugins) to install
-  --spatialite              Enable SpatialLite extension
   --version-note TEXT       Additional note to show on /-/versions
   --title TEXT              Title for metadata
   --license TEXT            License label for metadata
   --license_url TEXT        License URL for metadata
   --source TEXT             Source label for metadata
   --source_url TEXT         Source URL for metadata
+  -n, --name TEXT           Application name to use when deploying
+  --force                   Pass --force option to now
+  --token TEXT              Auth token to use for deploy (Now only)
+  --spatialite              Enable SpatialLite extension
   --help                    Show this message and exit.

docs/plugins.rst

@@ -258,3 +258,12 @@ you have one:
         return [
             '/-/static-plugins/your_plugin/app.js'
         ]
+
+publish_subcommand(publish)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This hook allows you to create new providers for the ``datasette publish``
+command. Datasette uses this hook internally to implement the default ``now``
+and ``heroku`` subcommands, so you can read
+`their source <https://github.com/simonw/datasette/tree/master/datasette/publish>`_
+to see examples of this hook in action.

docs/publish.rst

@@ -36,6 +36,8 @@ You can use ``anything-you-like.now.sh``, provided no one else has already regis
 
 You can also use custom domains, if you `first register them with Zeit Now <https://zeit.co/docs/features/aliases>`_.
 
+.. literalinclude:: datasette-publish-now-help.txt
+
 Publishing to Heroku
 --------------------
 
@@ -51,6 +53,8 @@ This will output some details about the new deployment, including a URL like thi
 
 You can specify a custom app name by passing ``-n my-app-name`` to the publish command. This will also allow you to overwrite an existing app.
 
+.. literalinclude:: datasette-publish-heroku-help.txt
+
 Custom metadata and plugins
 ---------------------------
 
@@ -71,9 +75,6 @@ You can also specify plugins you would like to install. For example, if you want
 
     datasette publish now mydatabase.db --install=datasette-vega
 
-A full list of options can be seen by running ``datasette publish --help``:
-
-.. literalinclude:: datasette-publish-help.txt
 
 datasette package
 =================