Use vim-gitgutter

ref. https://github.com/airblade/vim-gitgutter
Add key bindings for enabling/disabling GitGutter
Show gitgutter 2 left-side columns always
Revert 3 commits

Revert "Show gitgutter 2 left-side columns always"
Revert "Add key bindings for enabling/disabling GitGutter"
Revert "Use vim-gitgutter"

This reverts commits;
  7adcbef1308a753da89ca98f1074a919e5487b4d
  d2ee650cdd84dee3f52f5dcb18b86bbe9aa64992
  0832f4d24a6025cfcd81dee0e4d3c7a47250282f
Merge branch 'vim-gitgutter'