Create pluggable socket system

[baransu]

Resolve https://github.com/erlanglab/erlangpl-ui/issues/8

@arkgil you can find example how to create handlers in src/index.js. You have to import on function from src/sockets.js and return it as handlers for given plugin. Then you have to add it in combineSockets.

[arkgil]

This looks perfect, thanks! 👍

[baransu]

I'm polishing internals and then I'll merge