merge coverages instead of combining them (#1329)

[myzhan]

I copy gocovmerge to ginkgo, because it hasn't versions, and the code is easy to maintain.

[onsi]

Sorry for the delay - thanks this looks good to me. I only have one request, can you include the original license:
https://github.com/wadey/gocovmerge/blob/master/LICENSE

by adding it to the comment at the top of the new gocovmerge.go file? It’s a BSD-2 simplified license so I believe the two are largely compatible but I think it’s best to list it explicitly and make sure Wade Simmons gets the copyright notice.

@wadey please let us know if you have any concerns!

[onsi]

perfect, thanks!