Fix simplecov rcov calculation

Turns out #222 didn't really fix #153 

Basically, 
- SimpleCov divides lines into three states: covered, missed, and ignored.
- RCov divides lines into two states: covered, missed

The RCov formatter, being faithful to the RCov format, didn't annotate 'missed' lines, which #222 did not address, and hence #153 was not fixed.

In this PR, I introduce a syntax '--' to the RCov report generator that we then parse as a 'missed' line.  And all is good in the world.  Now, our RCov output is no longer really RCov, and in fact true RCov docs may be processed incorrectly as RCov covered lines were identified by the absence of '!!'. See logic in RCovFormatCoverage#assemble_files

All of this leads to the need to make a stronger break from pretending to use RCov, and to, at least 

1. Output the simplecov data in _data/ as `"#{MetricFu.report_id}_coverage.json"` or some-such that 
2. Is automatically picked up by metric_fu when running a report with that id.  (The report_id is really just YYYYMMDD)

That is, to no longer output an 'rcov.txt' to 'coverage/rcov/rcov.txt'

I'd love to change the metric namespace to TestCoverage.. but that's more work to do without breaking the existing contracted behavior.