switch from antigen to manual plugin management

- Took only what I actually used from oh-my-zsh and added to relevant
  plugin file located in zsh/plugins
- Took completions from oh-my-zsh and put them in zsh/completions
- Other plugins (autoenv, zsh-history-substring-search, and
  zsh-syntax-highlighting) are added as git submodules
vim updates
miscellaneous git, ag, and tmux configs