DateMath: Added ago/from triggers

Solved https://github.com/duckduckgo/zeroclickinfo-goodies/issues/879
For these tests, I'd probably prefer that MockTime is used, as it saves having to essentially re-implement the functionality in the tests. We've got some examples here : https://github.com/duckduckgo/zeroclickinfo-goodies/blob/master/t/CalendarToday.t#L87