Add Figwheel configuration

[ajoberstar]

Add a Figwheel plugin that adds an extension to ClojureScriptBuild with figwheel options. This won't provide any functionality within the plugin itself, but will be exposed over the tooling api to clojurephant-tooling.

clojurescript {
  builds {
    dev {
      figwheel {  
        openUrl = "http://localhost:8080"
      }
    }
  }
}