Colonfix

If there are no drive letters defined, then this plugin is supposed to do nothing.

This addresses Issue #4. Also adds three tests relevant to this bug.
I see. I guess the better test would be to put your test input into p_get_instructions() and check if the instruction contain a call to your plugin or not. The tests should also cover positive tests (eg. when drive letters are set up).
BTW. https://www.dokuwiki.org/devel:parser might help you to understand how the parser works a bit better