Check to see if zsh is even used

Last commit (9569441) was breaking my vim.  Whenever I try to start vim it just would segfault. 

$ vim
[1]+ Stopped vim

If I take out this change by removing the "\ -i" vim starts up fine.

I don't use zsh so not sure if that has anything to do with it.  My pull request checks to see if the user is even using zsh by checking the presence of a .zshrc file.
Merge pull request #87 from xiaogwu/patch-1

Check to see if zsh is even used