Feature request: go to cell below on enter, next cell on tab

I was expecting it to go to cell below on pressing enter and go to cell to the right on pressing tab. This is how spreadsheets normally do it, no?

(One weird thing I noticed, which made me think it's already supposed to work like that in #35, is that it does work like that if you don't add any `onChange` listener to the `Input`. )

Start by reproducing the behavior with an Input that has an onChange listener, then compare it with the behavior described in issue #35. Trace the keyboard handling for Enter and Tab. Done means Enter moves to the cell below and Tab moves to the cell on the right without breaking onChange behavior.

Written by the indexing model from the issue text.