Allow HTML summary to only show failed tests.

[cvanelteren]

The docs state

Summary reports can also be produced when generating baseline images and hash libraries. The summaries will be written to the results directory. When generating a HTML summary, the --mpl-results-always option is automatically applied. Therefore images for passing tests will also be shown.

However, I believe it would be beneficial to extend the option to only show failed tests. Is this already possible or could this be added to the pytest-mpl?