ENH: Make it possible to measure coverage of benchmarked module

[astrofrog]

It would be nice to provide a way to use asv with the coverage package in order to measure how much of a code base is covered by benchmarks. This is made a little tricky by the fact that asv runs the benchmarks in subprocesses, but it looks like coverage does provide a way to measure coverage in subprocesses

[HaoZeke]

That's an interesting idea! Is the plan to also have this integrated into the dashboard somehow (and therefore be part of say, the results object in some way)?