delete action shifted to right

similarly to https://github.com/plotly/dash-table/issues/64 when deleting the content of a cell, the content of a cell to the right of the selected cell is deleted.

![del](https://user-images.githubusercontent.com/2789078/45328108-60abef00-b528-11e8-8eb9-d0372f04b1e4.gif)
