Support loading plugin from ES Module URL

[oeway]

Suggested by @thewtex , it would be cool to load a plugin directly from a ES Module URL which exports setup and run.

I guess in addition to setup and run, we will also need to export an config object containing the meta info of the plugin. For security reasons we should perhaps use a sandboxed iframe to import the module, then inspect what's the plugin type in config, and then decide whether we should leave it in the iframe or move to a web-worker.