Changes to allow backward compatibility with plain ole CodeMirror themes

Loading theme files directly instead of a package breaks old themes; e.g. vanilla codemirror themes.  The issue is caused by adding the file extension as part of the theme name.  This logic was to handle edge cases when themes had the same filename but different extensions, but this edge case should really be resolved by giving files a better name.  This was important in earlier impls of themes that provided an interface to load all the theme files in a diretory (loadDirectory) but that interface was removed a while ago.
Ah, and what about double extensions like `.css.min`?
We should maybe add an unit test for that, too.