Move WebIO provider into Blink

[twavv]

Blink already depends on WebIO (in the sense that it's in the project's requirements) and the provider code in WebIO relies on hacks to properly display in Blink.

I'm happy to make a PR but would probably need some guidance.

[twavv]

Looking at the Blink provider code in WebIO, the following comes to mind.

We need a connection to use to communicate between WebIO and Blink.

Currently, the connection is instantiated whenever a WebIO thing is displayed in the frontend and uses Blink's preexisting messaging infrastructure. We could do something similar and just create a new BlinkConnection (would probably be renamed to WebIOConnection if moved into Blink) every time a WebIO node is body!-ed, but we could also just have it be per-page. There's not a lot of overhead in the current way of doing things so it might not be worth trying to make this better.

Consistency between Julia and JS interfaces

I actually started writing this section thinking that this was going to be very hard - but it's not.

Currently, WebIO bundles the requisite JS for all the providers which makes it easy to keep in sync when things change in how data is communicated between Julia and the browser. Essentially, Blink should just maintain it's own provider JS; this is very simple - all it does is tell WebIO how to send messages (i.e. using Blink.msg) and redirect relevant messages to WebIO (i.e. define Blink.handlers.webio).

[twavv]

This is done (will be really really done with #206 :'))