make ~~ redirect pattern non-greedy (closes #5)
honour conf['useheading'] for redirect note (closes #6)
basic support for external redirects

This commit adds basic support for redirects to external
pages, i.e. links starting with `http(s)://`.

A nice future enhancement would be the addition of support
for arbitrary DokuWiki links, including InterWiki and media
links.
Merge branch 'fix-useheading'
Merge branch 'feature-redirect-external'