Draft code for gallery filtering

[jukent]

Let's create a branch with fake code of how we'd expect a mystmd implementation of gallery filtering to work.

[brian-rose]

Relevant to design of new filters: ProjectPythia/cookbook-gallery-sphinx#223

[brian-rose]

It would be great to be able to filter the gallery on nightly build results, so we dynamically sort into "passing" and "failing". This would be very handy for maintainers, but also I think for users who might prefer to browse only through content that is currently operational.