Custom theme 'wook' on top of prezto zsh framework.

* 'dotfiles/zsh' is now sym-linked into '~/.zsh'.
* Enables git and vi-mode modules(plugins).
* Enables custom-located zsh script to work with prezto.
* Custom prompt theme 'wook', based on sorin's theme,
  is added and configured by myself. (more to improve)
Support <Ctrl-R> history search key bindings

'key-binding.zsh' defines the bash-compatible behavior
on <Ctrl-R> (backward history incremental search).

Now zshrc sources all the after-prezto zsh files
located in the directory 'zsh/zsh.d'.