Pass non-standard admonition categories on in HTML output

[mortenpi]

Arguably fixing a regression, although it is an undocumented "feature". But it is a good idea to pass these on.

Compared to pre-0.24 behaviour, the class names are now prepended with admonition- is-category- (not using is-, since that can conflict with various Bulma classes, like is-hidden) and fully sanitized (the final class names match [a-z0-9-]+).

Fix #1279.

[kescobo]

Thank you!

[kescobo]

Works like a charm:

From

.admonition.is-category-practice > .admonition-header {
 background-color: rgba(152,78,163,0.8);
}

.admonition.is-category-practice > .admonition-body {
 background-color: rgba(152,78,163,0.3);
}

Would you be interested in a PR to actually add this to the docs?

[mortenpi]

I am slightly hesitant to have it documented as public API just yet, in case we still want to make some changes to the structure of CSS classes. But if you don't mind, we could put the example with a few words of explanation into the example build (among other things, the example build functions as sort of a gallery of unofficial Documenter hacks).