Add code coverage adapters

[epompeii]

Some users have indicated that they would like to track their code coverage metrics with Bencher.
There are already dedicated tools to do this like CodeCov and Coveralls.
For open source projects on their supported platforms, one of those dedicated tools may be a better choice.

However, I can see the appeal of just using Bencher for this. There is nothing stopping users from tracking code coverage with Bencher. They just have to output Bencher Metric Format (BMF).
To make this easier, add built in adapters for popular code coverage tools.