DateMath: Added ago/from triggers

Solved https://github.com/duckduckgo/zeroclickinfo-goodies/issues/879
@W25 I'm not convinced that's actually true; I've just added a test for that and it appears to be parsed correctly and return the result I'd expect: https://github.com/duckduckgo/zeroclickinfo-goodies/commit/561bb233ae3ee93075e2a4e32e937fa3c642ee80