Conversion to both PDF and CBZ

[halvardssm]

Hi! Testing out this repo at the moment and I am loving it so far!

I wanted to make a feature request for that the conversion argument can be an array. The behavior would be as such:

• If not given, it will be the same as the current None
• You can pass in both pdf and cbz (and potentially future ones as well)

Thank you for your work!