Support for ignoring untracked files for git status

on oh-my-zsh is a switch you can enable if you want to disable marking untracked files as dirty.
this is a port of the switch
oh-my-zsh also claims that this makes repository status check for large repositories faster.

it's a 1:1 port, so i kept the name of the variable. to enable this just add 
```
export DISABLE_UNTRACKED_FILES_DIRTY=true
```
to your .bash_profile