[Plugin] sync-watch: Edit locally, run in the cloud

[Cory Johns]

Last week, while in Nuremberg, Ben and I were able to create a new
plugin to enable a much better charm development workflow.

Once the Juju Plugins bundle (https://github.com/juju/plugins) is
installed, you can now run `juju sync-watch <unit>` to enable local
editing of a remote charm.  This works using inotify to watch for
changes in the local copy of the charm and automatically sync those
changes up to the deployed unit.  If the unit is in an error state, it
will also automatically retry the failed hook, so that you can do
fast, seamless iteration on a charm.

This plugin works with any provider and any editor.