Support test coverage with instabul and nyc

This PR adds support for test coverage.

Things that can improve in this PR, but I still couldn't get them to work (All relate to instabul/nyc configuration):

Keep supporting devtool: source-maps instead of using them inline (So that it's easier to provide the lightweight version).
Ignore node_modules and webpack (and what else *.js). I still couldn't get normal exclude/include to work with the source maps.

Let me know what you think about adding this test coverage support. Any help is appreciated on the points above.

krasimir / webpack-library-starter