Added basic zsh support.
Added basic zsh support.

* Use shift instead of ${FOO:2}.
* Have the ruby command output env variables which we then eval.

regularfry/chruby,regularfry/chruby,postmodern/chruby,ngpestelos/chruby,ilikepi/chruby,postmodern/chruby,eunomie/chgo,kbrock/chruby,rstacruz/chruby-fish,JeanMertz/chruby-fish,kbrock/chruby,JeanMertz/chruby-fish,ngpestelos/chruby,rstacruz/chruby-fish,eunomie/chgo,ilikepi/chruby