add vim-gitgutter module
Multiple changes; see log entry

* Merge 'scripts' repository into dotfiles
* Add installer and updater for scripts
* Refactor determine_action(), link_file(), replace_file()
  to support types 'script' or 'dotfile'
* Add global update action (updates vim bundles, oh-my-zsh repo)
* Reverse verb order (s/zsh:update/update:zsh)
add script files