Recognize seconds in time calculations

[kfdm]

I wanted to calculate something like this

08:45:30 - 08:43:14

But it gives an Invalid Date/Time

I'm willing to try to commit a pull request if you could give me a few pointers on what I would need to change.

Looking at the parse_command I thought I could just change the TIME_MAPPING settings but it seems like perhaps more would be required.