Only the new cache_safe_extras option, with tests

[kamilkrzyskow]

Hi, 👋
as discussed in #19, I'm splitting the PR into multiple ones.

This PR adds the new option and a version bump. I preserved the structure and most of the code from the previous branch HEAD, so it's easier to see what was added and where for this option alone.
Apart from the option, tests were added to make sure that the old code works the same way and the new code works properly. I might remove the old tests in the future, as they are redundant currently, but kept them for now to limit the amount of removals in this PR.

I'll make more changes in another PR later.

[kamilkrzyskow]

Sorry for the forced push, I forgot to git add the new .yml files for the tests, so I decided to revert and fix it, to keep it in 1 commit.

[wilhelmer]

Thanks for your efforts, looks good to me! I think that we should find a better name for the option though.

We never call the minified files "extra" files. I know there's extra_javascript and extra_css in MkDocs, but we don't process these lists directly. Instead, the files must be listed seperately in js_files and css_files (which is useful IMHO, because you might not want all extra files to be minified, or you want to minify files that are generated by other plugins or scripts).

So cache_safe_extras could be a bit misleading; users could assume that all entries in extra_xy were hashed, which is not the case.

Suggestion: cache_safe

[kamilkrzyskow]

@wilhelmer
If the files aren't in the extra_ list then they won't be added to the html 🤔, and are ultimately useless, because they won't get linked to the page at all. However, I agree that the _extra could confuse the user and make them think they don't need to add any extra_ entries apart from the internal list.

[wilhelmer]

Yes, the minified files are a subset of the extra files, so you could say they are still extra files. Nevertheless, I think it's better to distinguish between the two in terminology.

I'll wait for the other PR and merge both if everything looks good. Thanks again for your contribution.