Add support to exclude files on coverage report generation

[henriquemotaesteves]

Add support to exclude files on coverage report generation (istanbuljs/istanbuljs#275) to nyc (#956).

[coveralls]

Coverage increased (+0.1%) to 96.524% when pulling 774d355 on henriquemotaesteves:issue-#956 into 12b8986 on istanbuljs:master.

[coreyfarrell]

I'm not very familiar with the istanbul-api module, I've requested that @bcoe also review this. I'm a bit busy right now but hopefully I'll be able to look at this soon.

[JaKXz]

@henriquemotaesteves sorry about this, would you mind rebasing this PR?

[bcoe]

This looks reasonable to me. One question that comes to mind though, is what advantage to we get by pulling in istanbul-api ... I always felt like istanbul-api felt like a weird abstraction, what does it add over using istanbul-report directly, and adding the exclude logic to istanbul-report ... just wondering what the value is of the extra layer of indirection.

Not a blocker for me though, just a question -- did you find you liked the api better exposed through istanbul-api, vs., interacting with the report module directly?

[bcoe]

@henriquemotaesteves looking good to me once you rebase and tests pass.