Add leading zeroes on metadata

[alfablac]

Problem:
ComicRack for Android sorts by issue number.
This end up sorting files like 1, 10, 11, 12, ... , 2, 20, 21, ....
Couldn't find any setting on Comicrack that fixes this, replacing all nubmering by padded numbering.
It would be awesome to have this automated inside comic-vine-scraper.