Add dependencyGraph plugin with sugar from Gilt

@schmmd this adds this nicer version of dependencyGraph:

https://github.com/gilt/sbt-dependency-graph-sugar

Note: this plugin could alternatively be added to your ~/.sbt/0.13/allenai.sbt and would be available to all projects without adding settings (this is what I had set up locally). This is in contrast with [the plugin that this wraps](https://github.com/jrudolph/sbt-dependency-graph) (which is the one you talked about in the engineering meeting).