Ex-vi (traditional vi)

Hi, this is the same pull-request as #4928.

The situation is now different and I had no problems installing the formula or running the program. I'm on OS x Mavericks.
Btw. this was tested with TERM env variable is set to `xterm-256colors` and there were absolutely no problems (unlike the issue in the original pull request).

A few arguments for including this formula:
 - vi is a classic piece of software
 - installing vi should be easy, but unfortunately it isn't (because of the lack of knowledge). I've even started a discussion on [/r/vim](http://www.reddit.com/r/vim/comments/1u0q0b/how_to_install_traditional_vi_not_vim_on_os_x/) on how to install it. There were a lot of misleading answers. The only fully working advice was to install ex-vi via macports!
 - `ex` and `vi` are symlinked to `vim` on OS X. It's nice to have them separated, so when I mess up vim, there's fallback