Compatibility with skip_covered

I have an existing project that I would read the coverage report with my human eyes. This project has `skip_covered = true` in the configuration, which causes files that are 100% covered to not show up in the report, thus an error when I try `:HighlightCoverage`.

How would you feel about a PR to add `--no-skip-covered` to the invocation inside `run_coverage_report`, or in the default provided by `find_coverage_script` in case the script isn't _just_ coverage and people would like to override to remove the flag?

Start by reading the run_coverage_report and find_coverage_script entry points, then reproduce the :HighlightCoverage error with skip_covered = true. Determine where the coverage command is assembled and whether the no-skip-covered option should be added there or exposed through the default script; done means fully covered files can be reported without breaking script overrides.

Written by the indexing model from the issue text.