Also, add support for updating plugins which keep gem versions in
files at the root of the repo (discourse-prometheus) and replace the
`up-plugin.sh` script with a README file pointing to the plugin
packaging documentation.
Some discourse plugins have Ruby dependencies and require a
specialized builder. This introduces a generic builder that can be
used whether the plugin has Ruby dependencies or not. It also adds a
set of pre-packaged plugins available through `discourse.plugins` and
provides an easy way to add more.
