Improve coverage tool for P3 Analysis Library integration

[Pennycook]

This PR introduces a new standalone cbicov tool that reads a compilation database and produces JSON that can be read in by the P3 Analysis Library. It is intended to be invoked as below:

cbicov -S <source directory> compile_commands.json coverage.json

This might need a little more work before it is ready to be merged, but I wanted to give us a place to discuss the functionality while we evaluate options for an automated Code Base Investigator + P3 Analysis Library pipeline.

[Pennycook]

@swright87 - I've added -x support, as per your request. It should work the same way as it does in codebasin, but please let me know if you encounter any surprising behavior.