remind: add support for date in .at command

[Exirel]

This PR is based on #1581, and it replaces PR #1162.
Also fix #1148.

It can replace PR #1581 if someone is brave enough to do the review of all the changes in one go.
I added an extensive test-suite to make sure the remind .at command works as intended.

I made .at opinionated about how date are expressed, with these choices:

• YYYY-mm-dd is valid, as dd-mm-YYYY
• dd-mm-YY is also valid
• screw mm-dd-YY (or mm-dd-YYYY) format
• dd-mm is valid too, as mm-YYYY and YYYY-mm
• but there is no such thing as mm-YY or YY-mm
• date separator can be either -, ., or /, and it must be the same between each parts

Even with all the unit-tests I added, I spent a good one hour with a live Sopel instance, and everything went according to the specs.

[Exirel]

Wouhouh, tests pass with Python 2.7! That was my main concern at this point, given the extensive test suite I ran with Python 3.

[dgw]

Some little things from going through the single commit unique to this (not pulled from the base branch in #1581). Very few little things, in fact, but an important class-design consideration did come up.

[dgw]

Would like to merge this hot on the heels of #1581, so I rebased it myself to get it ready for review sooner. @Exirel if you would rather I didn't do such things in the future on your PRs, just say so. :)

[dgw]

I wanted to make this just a "Comment" review, but there actually are enough things I would like to change that it warrants making that official. It's really close, though! I think we can merge this tomorrow. 🤞

[Exirel]

Uh oh... apparently I made a mistake: since you forced-push to that branch, I had merge conflict locally for some unknown reason, and I think I messed-up the squashing... :-/

[Exirel]

After double check, I believe it's all good. Ready for a new (and hopefully last) round of review before merge.

[dgw]

This looks good now, and I hope it's not because I'm tired. 😹

Just one line note that can be addressed if @Exirel has time to do so before I merge this and agrees that it's worth doing.

[Exirel]

@dgw done