Allow setting custom rules

[killy971]

Add support for custom rules such as the one introduced by #3 to mark otherwise as fully covered.

[jml]

From what I can tell, the way to do this with hpc is to:

• create an overlay using hpc draft
• edit that overlay to say how you want coverage to go
• turn that into a tix file with hpc overlay
• combine the tix file with the other coverage output

(see https://wiki.haskell.org/Haskell_program_coverage#Example)

I think the way I want to do this is to keep the overlays in version control and then run hpc overlay and hpc combine as part of my build process.

What I don't know is how to do this such that hpc-coveralls sends the combined output to coveralls. My own experiments are complicated by my use of stack (#47) and multiple packages (#51).