Diff doesn't show untracked files

[firefueled]

I'm thinking this would be an useful feature for verifying changes about to be staged, instead of staging and then calling the "diff staged" command.
One could simply open each untracked file but the diff offers everything in one page.
This behavior could be enabled with a diff_untracked_files option, and although the commands necessary to make it work do alter the workspace, they are reversible.

I'm working on it right now and I could submit a PR soon. Would that be acceptable?