Automatically detect and build test coverage report from data in MetricFu data directory

[bf4]

Depends on #227

When metric_fu is run on 2014-05-04
And there is a coverage report in the data directory with the matching report id (20140504_coverage.json)
Generate a test coverage report for this run

This is a new feature, as currently, an external file would need to be set for coverage to run. Because #227 will add coverage report data that can be safely automatically linked to the metric_fu report, we may just read it in and process it.

See optional considerations at the end of #227