Filename timestamp handling is poorly designed

[bgutter]

In a perfect world, we would use a flexible filename parser that can "figure out" the format of dates in filenames. At the very least, we should default to an ISO format (or, the most "standard" thing that we can use in a POSIX filename).

The current default filename regex is weird, and the mechanisms for modifying it are super limited. For example, you're boned if your filenames are stamped with a 24-hour time.

I don't want to spend /too/ much time working on this, since major effort would probably best be spent just doing this all in elisp. But, I think this could be quickly improved.