improve extension parsing logic

[figsoda]

the two new functions are extracted for the --format feature I'm working on

this fixes the issue where ouch d ..gz and ouch d ...gz tries to delete the current and parent folder respectively, which can be very dangerous with --yes, the user will now not be allowed to do that as if the file doesn't have an extension

[marcospb19]

Cool! I'd never noticed that ..gz and ...gz could be problematic.

[marcospb19]

This fixed an issue a friend had where decompressing file.cbz was impossible, because Ouch doesn't detect any known extensions but detects the ZIP format via MIME types, previously it would think the output folder should have the name file.cbz, replacing the input by the output.

[marcospb19]

I've added this to the CHANGELOG in the bugfixes section for 0.4.1: Fixed overwriting archive file when decompressing archive detected with MIME types.