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/heroku.py

@@ -0,0 +1,99 @@
+from datasette import hookimpl
+import click
+import json
+from subprocess import call, check_output
+
+from .common import (
+    add_common_publish_arguments_and_options,
+    fail_if_publish_binary_not_installed,
+)
+from ..utils import temporary_heroku_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",
+    )
+    def heroku(
+        files,
+        metadata,
+        extra_options,
+        branch,
+        template_dir,
+        plugins_dir,
+        static,
+        install,
+        version_note,
+        title,
+        license,
+        license_url,
+        source,
+        source_url,
+        name,
+    ):
+        fail_if_publish_binary_not_installed(
+            "heroku", "Heroku", "https://cli.heroku.com"
+        )
+
+        # Check for heroku-builds plugin
+        plugins = [
+            line.split()[0] for line in check_output(["heroku", "plugins"]).splitlines()
+        ]
+        if b"heroku-builds" not in plugins:
+            click.echo(
+                "Publishing to Heroku requires the heroku-builds plugin to be installed."
+            )
+            click.confirm(
+                "Install it? (this will run `heroku plugins:install heroku-builds`)",
+                abort=True,
+            )
+            call(["heroku", "plugins:install", "heroku-builds"])
+
+        with temporary_heroku_directory(
+            files,
+            name,
+            metadata,
+            extra_options,
+            branch,
+            template_dir,
+            plugins_dir,
+            static,
+            install,
+            version_note,
+            {
+                "title": title,
+                "license": license,
+                "license_url": license_url,
+                "source": source,
+                "source_url": source_url,
+            },
+        ):
+            app_name = None
+            if name:
+                # Check to see if this app already exists
+                list_output = check_output(["heroku", "apps:list", "--json"]).decode(
+                    "utf8"
+                )
+                apps = json.loads(list_output)
+
+                for app in apps:
+                    if app["name"] == name:
+                        app_name = name
+                        break
+
+            if not app_name:
+                # Create a new app
+                cmd = ["heroku", "apps:create"]
+                if name:
+                    cmd.append(name)
+                cmd.append("--json")
+                create_output = check_output(cmd).decode("utf8")
+                app_name = json.loads(create_output)["name"]
+
+            call(["heroku", "builds:create", "-a", app_name])