Cell magic to simplify API

[ellisonbg]

David, so glad to see you working on this. I had looked at observable/Jupyter integration long ago but the observable parser and compiler weren't mature enough. I think it would be great to wrap the API in an IPython cell magic so a user could simply do:

%%observable
....

rather than writing the code in a string.

[davidbrochart]

Thanks Brian, that's a good idea. I'll try an implement that.

[davidbrochart]

Done in #7.
There is an example notebook.