Added GutterJsonOutput as a coverage service.

This allows seeing the raw gcov output next to your code inside Xcode,
using <https://github.com/neonichu/PuncoverPlugin>.
You should be able to use `coverage_file.coverage_data`, where `coverage_file.coverage_data[32]` is the number of hits for line 32, or nil if it's not a line of code, for example.