\timing interval

\timing is great.
\timing is helpful.
\timing has made me really good at mentally estimating numbers modulo 3600000.

Wouldn't it be great if we had a way of printing timing in more human friendly formats?

Attached is a patch that allows the following (new/interesting bits in bold):

As demonstrated, "interval" toggles to "off". There is no way to toggle to "interval".

I'm pretty flexible on how something like this gets invoked. We could leave timing alone and create a format variable. We could actually leverage the pre-existing interval-to-string code, etc.

Note: the current patch includes no doc changes. I'd figure I'd wait to do that after this patch or another gains some traction.