mix help release should show options added by plugins about exrm

[bitwalker]

Should be fairly straightforward. I'm pretty sure we can use the --help flag without conflicting with mix, but I'll need to double check. Assuming we can, we can have two help modes, one which is a general list of plugins and descriptions, pulled via the @shortdoc attribute, and one which provides help for a specific plugin via it's @moduledoc attribute. That's how mix works today.

[bitwalker]

@smpallen99 I've pushed this to master. You can now list out all active plugins with mix release.plugins and show help for plugins via mix release.plugins <name>. There are three attributes that drive the help info, @name, @moduledoc, and @shortdoc. They are all basically self explanatory, but name is used for giving your plugin a friendly name. If one is not provided, I grab the last namespace and downcase it, so ReleaseManager.Plugins.Conform would end up as conform.

Let me know if you have any questions! This will go out with my next release.