cabal init should include common options with GHC options, with -threaded and "with-rtsopts=-N" by default.

**Describe the feature request**
cabal init should enable a common options field, which is keyed to be picked by default, and that should include `ghc-options: -threaded "with-rtsopts=-N4"` by default.

**Additional context**
Just thinking, but figuring out how to get cabal to implement multiprocessing takes a bit of Google time (I've done it before, but this time it took me about 90 minutes to figure out how to get it working).

Having multiprocessing on by default increases convenience, and helps if, say, others in the community who teach, might be interested in teaching Haskell concurrency early on.

**Implementation Plan**

If accepted, give me about 2 weeks and I'll take care of it.