Adding sourcemaps for coverage reports

[magicmark]

html coverage reports currently show the compiled es5

Would be nice to either:

1. Add sourcemaps to istanbul reports
2. Switch to nyc and compile on the fly

(2 is maybe undesirable if you want the assurance of testing against the actual dist code. fwiw though, something like this patch does the trick.)

[ljharb]

If you'd like to submit a PR switching us to nyc, I think that'd be great.

[magicmark]

#63