derived ikiwiki major mode

[ihrke]

This patch creates a major mode for ikiwiki-markdown which is currently used to filter out ikiwiki directives (which look a little like markdown-links and are therefore erroneously followed by markdown-mode). Furthermore, link-following is ikiwiki-specific (see screenshot).

[kini]

You may wish to submit this to http://github.com/jrblevin/markdown-mode instead, since that's the official repo. This one is just a mirror.