Filter entries by entry tags

[lemon24]

Per roadmap. Enabler for https://github.com/lemon24/reader/issues/327 "get_entries(has_enclosures=...) should be a plugin".

To do:

• [x] code (counts and search too!)
• [x] tests (try to reuse the existing ones)
• docs
  • [x] API (maybe move the tags documentation to a type alias)
  • [x] user guide
  • [x] changelog