A different (and working!) approach to scalacOption injection

I looked at the implementation of `set every` in SBT, which I tried
and failed to use in the previous commit.

Instead, I added a command that lets us express this pseudo SBT:

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