Adding initial version of the Dart analyzer plugin.

Hey @devoncarew,

I know this is a large commit; don't worry if you can't review it right away :) I would like someone to look at it eventually though, as I'm not a Python pro and you wrote most of the other plugin so you know what's going on!

I'm putting this into a separate branch for now, so that I can iterate on it a bit to make it better before it goes into master. Mostly I want to add an option for enabling and disabling the analyzer if you don't want to use it :)

Thanks!