Clean up a long running script before running it again
Ref https://github.com/BohemianCoding/SketchAPI/pull/165

When you are working with a webview for example, the workflow becomes less than ideal: you need to restart sketch everytime you change something.

It would be nice if, instead of directly running the plugin, we could clean the previous one before running the new one.

This probably involves some changes in Sketch itself tho