add ability for %(airdate)s or similar in naming options

[breunigs]

If feasible, separate variables for year, month and day would be terrific.

[hreality]

Id love to have the airdate in my filenames.. it would make it far easier to organize IMO

[evliu]

bump :) I'd PR but lack of time to study through the current code and add it in, also because I'd just be messing around in a language I'm not too familiar with