Add the lektor-markdown-highlighter plugin

On IRC @gus suggested adding highlighting to markdown code blocks. Lektor has the lektor-markdown-highlighter plugin that enables syntax highlighting in markdown code blocks, and provides a get_pygments_stylesheet() macro for generating a pygments CSS file.

We haven't stopped symlinking the entire lego/packages directory (per TPA RFC 30), but enabling this plugin on every site won't have any effects unless the get_pygments_stylesheet() macro is used.

• build a source distribution from the 0.3.2 tag (commit b4425dc)
• verify the code in the source distribution is safe
• add to lego