Add addon.postBuild hook

implement a postBuild hook in the builder addons can use to run arbitrarary code after each build

This allows an added to hook into the build/rebuild process and return a promise that will be passed the results from the processBuildResult method.

My use case for this is that after every rebuild, I need to run `cordova build ios` on the command line but it does not affect the output of the js/css files so it needs to be after everything is done
@rjackson done