Plugins management

We had an issue with third party plugins installing things via apt while
already installing dokku itself via apt.

To resolve this, `plugins-install` and `plugins-install-dependencies`
now can take a `--core` flag allowing to execute the install hook only
on core plugins and not on third party plugins.

Another feature with this modification is the ability to disable/enable
plugins. Two new commands now exist:

* `plugins:enable <name>` to enable a previously disabled plugin
* `plugins:disable <name>` to disable an installed plugin. This won’t
  work on core plugins.

debian/postinst

@@ -16,7 +16,7 @@ case "$1" in
     sshcommand create dokku /usr/local/bin/dokku
     egrep -i "^docker" /etc/group || groupadd docker
     usermod -aG docker dokku
-    dokku plugins-install
+    dokku plugins-install --core
     rm -f /home/dokku/VERSION
     cp /var/lib/dokku/STABLE_VERSION /home/dokku/VERSION