Also have `--compiler` flag on CLI determine `--compiler-check`

Looking at the code, I noticed that the CLI is just used as an override for `compiler:` in the config / stackage snapshot. The `compiler-check` flag determines how strictly it's used. An unfortunate side-effect of this is that if you specify an exact version of ghcjs, it will ignore the 4th part of the version, which I suspect is happening in https://github.com/commercialhaskell/stack/issues/2436#issuecomment-237713600 .

How should this work? I think it should be based on the number of digits specified by the user in the version number. We could also consider using this for the compiler field within stack.yaml itself, though that's an iffier change.