Wip/h3nk3 shims

Moves the shim functionality from sbt-rc into Activator.

When generating project files there are multiple steps, some synchronous and some asynchronous. Therefore the implementation is sort of state machine (see generator.js). Lots of mutable state and other ugliness due to the fact that it is implemented in JS... The process still feels a bit brittle but I think it is good enough to start the discussions around.

Atm, the final step in the process does not work because of an issue with SbtConnector that I am investigating. Since the issue seems unrelated to this PR I want to go ahead with the PR anyhow to get feedback on what has been done. 

For review by @havocp and @Warry.

I gave the lookupKey a try but couldn't get it to work the way I wanted to. 

I don't think checking by running is bad though since if it does exist we just say "command run successfully" and end the process (see stream subscriber logic). So unless there's a specific reason not to use this approach I will leave it as it is now.