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

datasette/utils.py

@@ -1,6 +1,7 @@
 from contextlib import contextmanager
 from collections import OrderedDict
 import base64
+import click
 import hashlib
 import imp
 import json
@@ -376,6 +377,7 @@ def temporary_heroku_directory(
     plugins_dir,
     static,
     install,
+    version_note,
     extra_metadata=None
 ):
     # FIXME: lots of duplicated code from above
@@ -430,7 +432,8 @@ def temporary_heroku_directory(
                 os.path.join(tmp.name, 'plugins')
             )
             extras.extend(['--plugins-dir', 'plugins/'])
-
+        if version_note:
+            extras.extend(['--version-note', version_note])
         if metadata:
             extras.extend(['--metadata', 'metadata.json'])
         if extra_options:
@@ -876,3 +879,18 @@ def remove_infinites(row):
             for c in row
         ]
     return row
+
+
+class StaticMount(click.ParamType):
+    name = "static mount"
+
+    def convert(self, value, param, ctx):
+        if ":" not in value:
+            self.fail(
+                '"{}" should be of format mountpoint:directory'.format(value),
+                param, ctx
+            )
+        path, dirpath = value.split(":")
+        if not os.path.exists(dirpath) or not os.path.isdir(dirpath):
+            self.fail("%s is not a valid directory path" % value, param, ctx)
+        return path, dirpath