Renderer plugin support

[samuelson]

A way to add arbitrary rendering plugins, similar to the way the syntax highlighting works.

E.g. if this syntax is used @@@ render: dot actually render the code to SVG instead of highlighting.

The renderers could live in some pre determined directory and would be automatically triggered when showoff encountered that tag.

[binford2k]

The javascript to do such a thing should also be dynamically loaded as needed. That would let us include as many renderers as we want, without needlessly bloating the amount of data the viewer must download.