Added ability to open html in a new split.

The html feature is very convenient, but I find myself wanting to view
the source code at times. Without this option, I would have to open
thefile in a browser, right click and view page source.
Added support for converting to Mediawiki syntax.

My job uses Mediawiki based wikis. This allows me to continuing using
vim-notes and share my notes when my coworkers need to see them.
Updated documentation to contain :NoteToMediawiki

Also updated the documentation to contain the `:NoteToHtml split`
addition.
Added coloring to TODO, XXX, and strikethrough  to DONE
Fixed case matching.

The TODO and DONE should only be colored if they are exact case
sensitive matches. Also, only the first match should be colored.
Merge pull request #103: Open HTML in split & Mediawiki converter

In this merge commit I'm making a couple of minor changes:

 - The `---` sequence is now used as a horizontal divider in the
   Mediawiki syntax converter.

 - The changes to xolox#notes#html#view() created a temporary file even
   when the HTML was shown in a Vim split window, now the temporary file
   is only created when it needs to be passed to a web browser.

 - I changed some wrong indentation, removed some unused code (the
   counter variable) and renamed some private variables
   (s/hilight/highlight/g) and added word boundaries to the
   TODO/DONE/XXX matching in the file mediawiki.vim.

 - No trailing empty lines are added when a note is converted to
   Mediawiki syntax (there were in the pull request, but without context
   or explanation so I'd rather keep the Markdown, HTML and Mediawiki
   converters consistent).