Adds support to change the top level directory for plugins

[logicminds]

• previously by default, pluginator would only look in the directory named plugins. While this is a great notion, many existing plugin systems do not follow this path. This attempts to allow the user to change the default plugins directory to something different.

  Example: lib/plugins/my_group/plugin_name.rb can now be lib/alt_plugins/my_group/plugin_name.rb.

[logicminds]

I can't test locally due to #14 so not sure if the tests pass. Just wanted to throw this up for review.

[coveralls]

Coverage remained the same at 100.0% when pulling 92092594ebc940a7d670edddcede92badf1e45b4 on nwops:plugins_dir into e205d0e69334e3789f48f20c6c4d4f9b3fdaaf2f on rvm:master.

[mpapis]

great idea, I will try to provide feedback in next 9-10 hours