Support providing files and lines to include into report via json file

[petrochenkov]

The inclusion set may be determined by some external script, inspecting e.g. git history.

So, here we give some simple json format for such scripts to produce and for grcov to consume.

[marco-c]

@petrochenkov could you explain your use case?

[petrochenkov]

The inclusion set may be determined by some external script, inspecting e.g. git history.

^^^ This is the use case.
We need to collect coverage on a git diff, that is then post-processed by a script.
The post-processing is necessary because it mirrors a build system behavior. The build takes the original source code, then moves it around a bit changing some paths, then builds the changed source, so coverage contains the changed paths, not paths from the original source and diff. We do not control the build and cannot change it.

I initially wanted to support just collecting coverage from a diff (patch file format), but given that it's not enough, I implemented a more general approach that just eats an explicit dump of source ranges.