Incremental execution of code

Hello, I made a quick search and hope I didn't miss any relevant issue.

**Describe the solution you'd like**

I'm a big fan of live programming (REPL FTW :) and I had a great experience with atom + [hydrogen](https://atom.io/packages/hydrogen) and now I'm trying to find VSCode's parallel.
hydrogen works with the jupyter protocol under the hood, and allows you to run one line of code here, another over there, without the underlining "state" disappearing (i.e if I declared a variable I can refer to it on the following executions) this (IMHO) makes a great leap in the development experience towards the holy grail of a true conversation with the computer.
Not sure if it's even possible (or already possible) with the current implementation but was worth enough to bring the subject up for discussion.

Keep up the great work!