Toggling vimdiff for open buffers

Quick intro to vimdiff, including keymapping for toggline vimdiff with files already open in buffers. Also using vimdiff for all 'git diff' commands.

Correction: the keymapping should actually be `nnoremap <expr> <Leader>d exists('g:diff_on') ? ':windo diffoff<cr>:unlet g:diff_on<cr>' : ':windo diffthis<cr>:let g:diff_on = 1<cr>'`