DuckDuckGo: expand trigger-space.

- Add "duck duck go" trigger.
- Add possessive versions of triggers.
- Allow triggers to appear anywhere in the query.
- Skip prepositions and definite articles before key conversion.

Fixes #645.
DuckDuckGo: add spacing variations for 'duck duck go'.

Who knows how someone may type it, so we'll assume they mean us no
matter what craziness is involved.

Also, clean up the tests a bit such that text changes need only be done
in one place, while the triggering can be run in multiple places easily.
Merge pull request #646 from mwmiller/ddg_is_duck_duck_go

DuckDuckGo: expand trigger-space.