Use vim :TOhtml as fallback renderer

In the case where `GitHub::Markup` cannot render a given filetype, it would be good to use vim's built-in `:TOhtml` command instead.   This gives us the added capability of rendering *any* unrecognized file using vim's syntax highlighting!

The check for this capability should be really safe, as it is the same as used in the [TOhtml plugin itself][1]. (This also depends on my Pull Request #51 to work on Linux)

Screenshot
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![vim-Hammer-TOhtml-Screenshot](https://f.cloud.github.com/assets/122524/94681/98eed91a-6653-11e2-9860-23f81dee2602.png)



[1]: http://vim.cybermirror.org/runtime/plugin/tohtml.vim