Should be able to define/specify one or more "plugin directories" from which to load plugins during bootstrap

In order to support custom plugins, and bundling plugins from multiple sources, Forge should accept a system property:

-Dorg.jboss.forge.pluginDirs=~/.forge/plugins,/opt/forge/plugins,etc...

Allowing one or more plugin directories to be specified. If no directories are specified, then we can either default to ~/.forge, or disable the plugin mechanism entirely.

I would lean toward defaulting, because disabling plugins might be more work and not really beneficial.