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.
This bug caused the link to not appear at all. So by checking for the title, this test fails if the bug is present and succeeds if the bug is fixed. Also a simple title should be changed by any refactoring later in the lifetime of this plugin and this test is therefore unlikely to report false-positives.

However checking on the link itself or other tags in the `<a>...</a>` string might create falsely positive tests when something changes with the handling of links within dokuwiki itself.

Also I am not sure if `[[:facts:figures|Foo]]` and `[[facts:figures|Foo]]` are actually supposed to created the exactly same link or if the will always be supposed to do that in the future. Or how `[[\facts:figures|Foo]]` and `[[/facts:figures|Foo]]` are supposed to be handled now and in the future.