Add cordova run option to skip prepare

The 'cordova run' command currently always does a "prepare" operation for the target platform before launching the app. There is a --nobuild option (to skip building the native project), but there isn't a --noprepare option.

This option will be helpful to optimize build-and-run scenarios in the Visual Studio tools for Cordova. VS always ensures the project is fully prepared and built before trying to launch. So by the time it calls 'cordova run' there is no need to do the prepare step, and it just causes the developer to wait extra seconds for every iteration of the inner loop.