Improve format entire file mapping.

This is better because:
1) applies the format faster!
2) it isn't jumpy and doesn't select-deselect the text
3) using `normal!` we obtain the same result even if the user remap some
   of those keys
4) avoid remapping (`nnoremap`)
5) `gg=G` is shorter than `ggVG=`
Merge pull request #538 from ivanalejandro0/improve-format-entire-file

Improve format entire file mapping.