[TW-1800] Add the ability to pause a recurring task.

_Alan Young on 2016-05-09T23:24:46Z says:_

Being able to pause a recurring task would be simpler ... and harder to forget.

Something like

task add due:... wait:... pause:... Some task.
task 1 modify pause:(same as due or wait)

I'm not sure how an indefinite pause would be done though.