Better key bindings for history substring search.

See robbyrussel/oh-my-zsh#2701 [1].

  [1]: https://github.com/robbyrussell/oh-my-zsh/issues/2701#issuecomment-39975056
Some new zsh options and bindings.

* Explain what AUTO_CD does
* Use INTERACTIVE_COMMENTS to allow # at the beginning of a line to act as a comment
* Use ^Q to push the command to the stack, part of the zsh line-editor [1]

  [1]: http://bolyai.cs.elte.hu/zsh-manual/zsh_14.html