Customization of test --coverage options

[mgsloan]

The fix to #634 makes test --coverage use a very sensible default. However, doesn't automate things for you if you want coverage reports that span multiple packages or include test-suite code.

One possibility would be to use the decisions from #652, and have a --hpc-options flag that specifies extra hpc options. This seems like a good thing to have in general, but TBH hpc's CLI is a bit unwieldy. So it may also be good to have some of the common patterns of usage wrapped up in some nice flags.

Another thing to consider here is the possibility of having a "unified" coverage report for your project - #579

[mgsloan]

It might be worthwhile to have a stack hpc command, which wraps the hpc program, providing it all the dirs it needs for .mix files. This would allow you to analyze .tix files that come from executable runs.

[mgsloan]

Note: in order to provide coverage reports for the test code itself, we'd need to implement a proper solution to #716.

Implementing this will also bring back #607.

[mgsloan]

Closing this out since most of the ideas here have been implemented, and the rest can go in more specific issues.