support REPL interface

[jaked]

the VS Code notebook UI has support for a REPL-style interface (where you type commands at the bottom and see a stream of result cells). I tried to hook it up following DonJayamanne/typescript-notebook@c9d395a but I can't get it to work: it won't select the Vitale kernel after creating the repl, and I can't figure out a way to debug into VS Code to see why the selectKernel command isn't working.

Work in progress on https://github.com/githubnext/vitale/tree/jaked/repl