New extension for gnuplot integration

Hi!

I just uploaded a new extension to generate plots via gnuplot within middleman. It is still at a basic level, but I’m planning to add more functionality.

It’s available at github: https://github.com/hermanndetz/middleman-gnuplot and via https://rubygems.org/gems/middleman-gnuplot.

I hope you’ll find it useful! Any feedback is welcome!

Hermann