Add options to change cover image kind and supplement

[LordBaryhobal]

I would find useful to have the option to change the cover image kind and supplement, if someone wants to put a CeTZ canvas for example

On the same topic, I find it quite restricting to panic if the figure type is not one handled by the function getSupplement (and no supplement is given). I have changed it to fallback on an auto value, which shouldn't make much of a difference, except not throwing an error.

[LordBaryhobal]

it seems like I have messed up and included a commit from my other pull request
I will try to fix this

EDIT: fixed it

[pmudry]

Thanks a lot for those changes. You are right about the panic, it's a bit too much but to be honest it was more for testing than for anything else. Makes sense though :)