Displaying covered lines as well

Just wondering if it might be possible to have something similar to vim-go's coverage:
- [goCoverageNormalText](https://github.com/fatih/vim-go/blob/00c5f2dad170131c0c850dbf331d63ddf515116d/syntax/go.vim#L389) greyed
- [goCoverageCovered](https://github.com/fatih/vim-go/blob/00c5f2dad170131c0c850dbf331d63ddf515116d/syntax/go.vim#L420) green
- [goCoverageUncover](https://github.com/fatih/vim-go/blob/00c5f2dad170131c0c850dbf331d63ddf515116d/syntax/go.vim#L421) red

I don't know much about `coverage.py`, just started using, so the limitation might be there, but asking in case we can enhance with that as it is quite useful.

The issue names no repository files or tests, so start by inspecting the plugin entry point and how coverage.py data is currently consumed. Compare the three vim-go highlight groups linked in the issue, and confirm that covered, uncovered, and normal lines are distinguishable in a Vim buffer.

Written by the indexing model from the issue text.