cell type "raw" not handled

[heseber]

nbterm breaks if a notebook contains cells of type raw. In the function set_input_readonly(), only cells of types markdown and code are used to set the text variable, so if a cell of type raw is encountered, the text variable will be uninitialized in line 181 of cell.py.

[davidbrochart]

Thanks @heseber, would you like to open a PR?