Easier interval splitting at given times

Currently there does not seem to be any easy way to split a part of an interval.

For example, if you have logged interval 12:14:45 - 14:25:36, and then remember that you got a call at 13:15-13:40 that you would like to have a separate interval for, it would be a pain in the ass to modify with current lengthen/shorten/resize/split.

`timew split-at [ ...]` could work like this:

timew split-at 13:15 13:40

For each specified `date`, the interval that contains that date would be split into two separate intervals. You could then modify the tags separately.

Start by locating the existing lengthen, shorten, resize, and split command implementations and the interval-handling code they use. Define the behavior for `timew split-at [ ...]`, including multiple dates, and verify that each containing interval is divided so the resulting parts can be tagged separately.

Written by the indexing model from the issue text.