#17 Configurable names for screenshots

[stoo]

I would really appreciate it if the filenames of screenshots contained the name of the current map and optionally a date/timestamp.

Something like E1M1_20200122_140425.png instead of just SPASM0001.png

If multiple shots are taken within 1 second then just append an incrementing number or whatever to them.

This would be very helpful for a project I'm considering,

I love QuakeSpasm. Thanks.