A different (and working!) approach to scalacOption injection

In the previous commit, I tried and failed to use `set every` to
append compiler options. It isn't the right tool for this job,
unfortunately.

But, I took a look at its implementation of in SBT, and figured out
how to roll my own.

The result is a custome command that lets us express this pseudo SBT:

  val releventScopes = scopes.filter(_.defines(scalacOptions))
  buildSettings += releventScopes.map {
    scope => scalacOptions in scope += extraOptions
  }