Support nbdime diff tool

Allow selection of diff program for jupyter notebooks.

When I diff my jupyter notebooks, I see differences in all of the irrelevant metadata and output, when all I care about is the code. It's really time consuming to go through notebooks I did not change, but still have changes from running them, to figure out whether I need to commit and push them.

A python tool 'nbdime' has been created to 'nbdiff' these files. I'd like to have GitKraken use that diff tool, but only for .ipynb files (I don't know if notebooks using R or bash have different extensions).