Add a test --coverage option

[erikd]

The stack tool has an option so you can run stack test --coverage and it will build the tests and run them collecting test coverage information and at the end report the path the a HTML file with results.

I haven't looked yet, but I do not think stack is doing anything more complicated than calling HPC (the Haskell Program Coverage tool).

Would be nice to get this working for mafia even if there is a change for this to be used for evil instead of good.

I'll have a crack at this on the weekend.