Unify prism script and CSS discovery routines
Add MIME type for UTI imports
Make analyser shut up about warnings in dependencies
Split up Prism scripts to reduce file size

Part of the implementation for #2
Make Prism theme configurable and fix script dep
Try to also include the “extras” variant
Merge branch 'prism-breakup'

Fix #2

Implement hooks for Prism plugins seems a bit like an overkill on second
thought. We are only building a Markdown editor, not a full-blown syntax
highlighter, after all. If the user wish to tweak the output HTML he/she
can always do so after exporting; we don't need to go *that* far.