ikiwiki/doc/todo/attachments_plugin.mdwn

First pass at an attachments plugin. See [[plugins/contrib/attach]] for details/docs. Here's the [diff](http://pastebin.com/f4d889b65), and here's some [technical notes](http://pastebin.com/f584b9d9d). There are still various things I want to fix and tweak, but it works reasonably for me as is.