Implemented multi-project support

The changes ended up being far simpler than I originally thought, I my lack of SBT experience showing through... I had to disable parallel build in order to prevent file write failures. 
I think that adding the `-a` flag is going to cause the tags file to always be appended to and never removed from. So as code changes, and `gen-ctags` is run again, I think there will be tags that linger around and point to old lines of code that no longer line up with the freshly generated source files.

Please correct me if I'm understanding this incorrectly.