enable local plugins

[salzig]

This change will allow you to place plugins into $LOLCOMMITS_DIR/.plugins (typically: ~/.lolcommits/.plugins) for experimentation or to use plugins that are not yet merged into lolcommits.

Every plugin placed into $LOLCOMMITS_DIR/.plugins has to implement the same interface as plugins shipped with lolcommits.

[salzig]

fixes: #99

[matthutchinson]

This looks good to me, and a nice approach to help move our existing plugins into separate gems (and experimenting with new ones).

I don't think we should close out issue #99 just yet, since we'd like to see some of the existing plugins as gems, and have them auto-loaded if they are installed, not just from this new .lolcommits/.plugins dir.

@mroth if you think this OK too, please merge away.

[mroth]

Agreed, this could be helpful as we move towards #99, but definitely shouldn't supplant it.

It's unfortunate that as far as I can tell there is no equivalent of npm link in the ruby gems world, otherwise I would say this PR would be obsolete once we implement #99 as the proper packaging method for third-party plugins, but given there isn't something simple like that, this could definitely aide development for plugin makers then without them having to constantly do gem install --local while developing.

[mroth]

Thanks @salzig ! Merging now.