Support `--html-single-page`

`gcovr --html-single-page --html-details coverage_report.html` combines all the `coverage_report*.{css,html}` into a single file, which is a bit nicer to pass around and download as an artifact.

Alternatively, provide an option to pass arbitrary extra arguments (`gcov-executable` allows it but requires to also pass `gcovr` path itself, which might not be known).