Plugin support: run the example app of a consumed plugin

When a Flutter developer adds a plugin to their pubspec, they are able to call the APIs in it's interface. However, to discover the plugin's abilities it would be very convinient to be able to run the /example/ app located inside the plugin.

Currently this is possible manually; but requires opening a second IJ window, and then File > Open'ing something like `~/.pub-cache/hosted/pub.dartlang.org/url_launcher-0.4.2+1/example/`