Fix rake new_post['title'] and Zsh

There is a bug with Zsh and the Octopress rake new_post command.

Everytime you try to create a new post using this command, you get this:

$ rake new_post["My new post"]
zsh: no matches found: new_post[My new post]

The only workaround so far is:

rake new_post\["My new post"\]