Add example of using custom plugin with webworker

[cmdcolin]

I believe this involves customizing the makeWorkerInstance function to, currently, be fully custom instead of importing the exported version. There are are couple options

a) Make a system where the makeWorkerInstance can accept custom plugins to add to the corePlugins
b) Make example of how a user can create their own version of rpcWorker.js and call new Worker('...') themselves

The challenges with (a) are that it is not really possible (??) to forward a user-passed plugin through the new Worker('...') syntax

The challenges with (b) is that it's a bit hard to instruct users how to properly use it, but it might be the most flexible solution