Error message for --format .tar.gz outputs empty extension

[marcospb19]

Version

0.4.2

Description

When running

ouch compress LICENSE output --format .tar.gz

Current Behavior

ouch outputs

[ERROR] Invalid archive format
 - Unsupported extension:

Expected Behavior

Properly handle the --format flag with the leading dot.

OR

Output this instead:

[ERROR] Invalid archive format
 - Unsupported extension: .tar.gz

[marcospb19]

I'll take this one after #515 is merged so I can add tests for it to avoid regression.

EDIT: actually now waiting for #518 to reuse some of its code.