Allow gcovr >= 3.1 to be used to generate html coverage report

Modern gcovr includes html generation support so if lcov and
genhtml are not available fallback to gcovr.

Kept lcov and genhtml as default so to not surprise existing
users of coverage-html with the different output of gcovr.

gcovr added html support in 3.0 but as there already is a test
for 3.1 because of the changes to -r/--rootdir I opted to only
allow html generation for >= 3.1 to keep things simple.

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