Add Config to make stashing to include untracked files automatically

Right now default git stashing behavior doesn't include untracked files in the stash. I am annoyed by this most of the time. I create stash then undo it and then create a new one and include the stash with untracked files.

My proposed solution is to make a lazygit config when enabled the default stashing uses `git stash -u` rather than `git stash` essentially making stashing untracked files the default behavior.

I tried to make this behavior with `.gitconfig` but there is no git config option.