Unit Composition plugin

Hopefully it's fairly straightforward about what the approach. I have tested it some (and actually looked at the replay to verify), but I'm thinking there are probably still edge cases out there to consider.

Graylin, I'm thinking that you probably have a better sense for what these edge cases are regarding unit types and such.

Hopefully this plugin is generally useful for others. I know other sites are already extracting compositions in their own way, but it would be kind of nice to have a standard method for doing it.