Investigate coverage support for yerbamate child execution

[angrykoala]

The scripts executed with yerbamate are not covered by a coverage tool like istanbul (as they are processed from a child_process). Investigate if is possible to get coverage information of those files.

[angrykoala]

This could be useful: https://github.com/istanbuljs/nyc

[angrykoala]

Also gotwarlost/istanbul#97

[angrykoala]

The tool nyc supports spawn coverage.

Executing tests using yerbamate with nyc as coverage tool should cover files properly:

Usage Example:

"scripts":{
 "test": "nyc --reporter=lcov mocha"
}

Disclaimer: using nyc heavily affects on performances when testing using yerbamate (around 2~4 times more for each test). Use only when coverage is required.

Update readme with this data.

Using nyc is up to the user, investigate options to integrate this feature with yerbamate

[angrykoala]

nyc is very slow when processing tests with children, if integrated with yerbamate, use as an option