expose full completion data to extraKeys in show-hints.

Hi, 

Accessing full completion data is convenient is one want to bind `extraKeys` in `show-hints` to insert the common prefix of all completions (cf [context in question I asked a few hours ago](https://groups.google.com/forum/#!topic/codemirror/tGRx65Sft70) ).

This is my attempt at adding what I need to have this functionality. 

I added some more doc at the same time. 

Hope you will accept or at least add this capability to 4.x

Thanks.