Add textobj-diff to the configuration.

Originally, I wanted something that recognized diff files and would allow `]]` and `[[` to move between hunks.  There is another plugin called diff_movement and a depenedency called CountJumps that supports this as well.  I didn't like the plugin as much though--it was a little too convoluted.

Kana had this plugin, textobj-diff, that was most of the way there.  It just needed the extra mappings to allow `]]` and `[[` to work.

The only question I have here are the movement mappings that textobj-diff brings.  It has 8 `<Leader>d` mappings and I'm just to sure we'll use them.  If we don't want them, we can drop this into the vimrc:

```vim
" -------------------------------------------------------------
" textobj-diff
" -------------------------------------------------------------
let g:textobj_diff_no_default_key_mappings = 1

function! s:SetTextobjDiffGlobalMappings()
    for m in ['x', 'o']
        let cmd = 'silent! ' . m . 'map '
        execute l:cmd . 'adH <Plug>(textobj-diff-file)'
        execute l:cmd . 'adf <Plug>(textobj-diff-file)'
        execute l:cmd . 'idH <Plug>(textobj-diff-file)'
        execute l:cmd . 'idf <Plug>(textobj-diff-file)'
        execute l:cmd . 'adh <Plug>(textobj-diff-hunk)'
        execute l:cmd . 'idh <Plug>(textobj-diff-hunk)'
    endfor
endfunction
call s:SetTextobjDiffGlobalMappings()
```

That will prevent textobj-diff from adding any mappings, and then add back in the ones for selecting the diff objects.  I can update the pull request with this change, if you think it's better to drop the leader mappings.  I'm pretty sure I'm not going to use them... just my 2 cents.