optional padding and sep

[seanth]

Extended to address https://stackoverflow.com/questions/75032116/pdf2image-how-to-remove-the-0001-in-jpg-file-names

includes pdftoppm option "-sep"

I wanted to remove the padding given to file names, as well as the "-" separator. The code changes let users have an option "-sep" arg that rolls into the pdftoppm args, and users can define the amount of padding

[Belval]

I have merged a lot of PRs today, just wanted to drop a message to say that I've seen this and I do think it has to be addressed, but I need to understand/test it more thoroughly to make sure it does not create backward incompatible changes.