Allow a dot in the default format

This allows the default format to be "tar.gz", etc (as is mentionned in the README) Before, as the default format was only looked for without a ".", formats such as tar.gz couldn't be used as default. For example: ```toml [[mgr.prepend_keymap]] on = ["A", "z"] run = "plugin compress 'tar.gz'" desc = "Tar" ``` -> Expected behavior: I can enter a filename, and it will automatically be used as the base of a `.tar.gz` archive -> Buggy behavior: it would just create a `.zip` file, as my argument was never parsed