Support loading of mixins

See https://github.com/strongloop/loopback-datasource-juggler/pull/201
Here is the meaning of different suffixes:

 - `modelSources` lists places where to look for models, not all files are required
 - `bootDirs` lists directories where all files are loaded
 - `bootScripts` lists a files that should be added to the list of boot scripts

To stay consistent with this convention, mixins should use `mixinDirs` and `mixinScripts`.