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/publish/now.py

@@ -0,0 +1,80 @@
+from datasette import hookimpl
+import click
+from subprocess import call
+
+from .common import (
+    add_common_publish_arguments_and_options,
+    fail_if_publish_binary_not_installed,
+)
+from ..utils import temporary_docker_directory
+
+
+@hookimpl
+def publish_subcommand(publish):
+    @publish.command()
+    @add_common_publish_arguments_and_options
+    @click.option(
+        "-n",
+        "--name",
+        default="datasette",
+        help="Application name to use when deploying",
+    )
+    @click.option("--force", is_flag=True, help="Pass --force option to now")
+    @click.option("--token", help="Auth token to use for deploy (Now only)")
+    @click.option("--spatialite", is_flag=True, help="Enable SpatialLite extension")
+    def now(
+        files,
+        metadata,
+        extra_options,
+        branch,
+        template_dir,
+        plugins_dir,
+        static,
+        install,
+        version_note,
+        title,
+        license,
+        license_url,
+        source,
+        source_url,
+        name,
+        force,
+        token,
+        spatialite,
+    ):
+        fail_if_publish_binary_not_installed("now", "Zeit Now", "https://zeit.co/now")
+        if extra_options:
+            extra_options += " "
+        else:
+            extra_options = ""
+        extra_options += "--config force_https_urls:on"
+
+        with temporary_docker_directory(
+            files,
+            name,
+            metadata,
+            extra_options,
+            branch,
+            template_dir,
+            plugins_dir,
+            static,
+            install,
+            spatialite,
+            version_note,
+            {
+                "title": title,
+                "license": license,
+                "license_url": license_url,
+                "source": source,
+                "source_url": source_url,
+            },
+        ):
+            args = []
+            if force:
+                args.append("--force")
+            if token:
+                args.append("--token={}".format(token))
+            if args:
+                call(["now"] + args)
+            else:
+                call("now")