Anagram: allow for multi-word queries.

This still only returns single-word anagrams, but the query can contain multiple words now.

Also:
- Fix up the query triggers to be more sure of intent. 
- Caching enabled by default; turned off for "scrambled" responses.
- Add some internal caching for letter frequencies of dictionary words.
- Removed the ability to request "anagrams" of length less than th length of the query
Looks like we currently support that triggering, so we might as well keep it https://duckduckgo.com/?q=anagram+filter