Allows a single file to run and output by default in overwrite mode.

The cli instructions imply that this works, however the tool function was missing the code for it.

Also I added a final else to make sure that an error is thrown if something goes wrong and none of the other if/ifelse catch.

If you try to run just a single file on the cli:

php csscomb.php -i any_css_file.css

The cli tool reports done, however nothing happens.

Expected Behavior:

any_css_file.css is overwritten with the sorted version.