Handle different schemes by default

[makew0rld]

The [url-handlers] section has a key called other, which defaults to a value of "off". There should be a new default value of "default", which will use the default opening command for each OS to open the URL (technically a URI).

Anyone with the old config will have to edit their config to access this feature, and this should be emphasized in the release notes.

This will require changes to the wiki, as it already has a page on this.