Remove excluded files from coverage before writing

[princjef]

Fixes #581 by scrubbing the coverage object to remove any excluded files for which coverage was generated.

I'll admit to not being entirely sure how the coverage gets in for those files in the first place (the exclude logic appears to be running correctly in addAllFiles()), but this provides a catch-all at the end of processing to clean it up.

There's also a question about whether the scrub should occur before or after following source maps. Doing it before seems to make the most sense based on how things work now.

[coveralls]

Changes Unknown when pulling d7dcc74 on princjef:exclude-all-fix into ** on istanbuljs:master**.

[bcoe]

@princjef very odd! seems like a reasonable safety to put in place though. I'll work on landing this as soon as possible.

Worth noting, I stood up a community slack for coordinating work on this project if you feel like helping coordinate work around getting another release of nyc out:

http://devtoolscommunity.herokuapp.com/

[bcoe]

@princjef I've released this for you in the latest version of nyc, mind testing things out? thanks for the contribution 🎉

[Speedy37]

This broke my builds, because I'm using NYC_CWD / --cwd to bypass /node_modules/ default exclusion in my tests base path, but I still want to exclude /node_modules/ after it

ie: I want /mybase_path/node_modules/mymodules/**
but not /mybase_path/node_modules/mymodules/node_modules/**

And this can't be expressed with -x -i even with -x "!/node_modules/" due to the way they are resolved

[Speedy37]

I spoke to soon, it is commit dd40dc5 that broke my build