Basic support for external redirects + fixes for #5 and #6

Until now, only redirects to DokuWiki pages have been supported.  This PR adds basic support for redirects to (external) URLs, following the same syntax:

``` 
~~REDIRECT>https://example.com~~
#redirect https://dokuwiki.org
```

Additionally, fixes for the following two issues are included as well (can be merged separately, if needed):
 * #5, see d77260f
 * #6, see a693d6b

If you prefer a linear history, I can rebase the individually merged commits onto the current master.