Added switch flag to use Snap install directory

[its-Winter]

These changes work like a charm for me just doing ./firefox_decrypt.py --snap
Adds -> #111

[unode]

@its-Winter thanks for the pull request but this is already supported by doing:

./firefox_decrypt.py ~/snap/firefox/common/.mozilla/firefox

[python312]

i think the merger wants this directory to be automatically scanned

[unode]

@python312 this is clear but given that the profile-directory can be passed as an argument, this can be easily solved with a shell alias.
No code changes are necessary.