Extract out RCovFormat handling logic
Include RCov css in RCov template, not all of them
Remove unused css from analyzed file report
Spec out MetricFu-specific SimpleCov formatter

Currently only rcov-format specific

See issue #153 for known bug

Adapted from https://github.com/kina/simplecov-rcov-text/
by @kina, William 'Kina' <thekina@gmail.com>
MIT-licesned