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. 
If I understand correctly, `ThisBuild` is the global SBT scope, so this will disable the parallel execution of _any_ task the user tries to run (even if not related to sbt-ctags). It's probably not too polite for this plugin to do that to people.

Fixing #3 would probably allow for something like `Keys.parallelExecution in SbtCtags := false`, which would be much more reasonable. However, the preferred solution is probably to find a way to allow these things to run in parallel without causing trouble (if possible).