Random theme selection from favourite list

As discussed in #482 i wrote this simple patch.
The only thing you've to modify your zshrc adding something like:

    export ZSH_THEME="random"
    export ZSH_FAVLIST="/home/fox/.zsh_favlist"

then omz will pick random themes from the favlist ;)
Maybe it could be done better but i'm not a zsh ninja

I added the q/quit stuff too