[TW-1459] Detect and warn about the mistake of putting a date in the description field

_Jeremy John Reeder on 2014-11-13T20:00:51Z says:_

A very common mistake (for me, at least) is to accidentally set the description of a set of tasks to a date. That is, when I intend to do `task proj:home mod wait:11-13T16:00`, I instead do `task proj:home mod 11-13T16:00`. There's a confirmation prompt, but I've gotten used to that prompt and nothing makes it obvious that I'm doing something totally wrong. So it would be helpful if Taskwarrior could detect that the description appears to be a date and say something to that effect, preferably in a color that indicates that it's a warning.