Hosts file auto-update

This piece of code will enable vagrant to auto-update the hosts file if
the hostsupdater plugin has been installed. 
If it has not been installed, it will not execute the code that wil add the domains to the hosts file.
(Supports: Windows, Mac OS X, Linux)

This takes out the fact that you manually have to edit your hosts file.

If this were to be accepted, you'd have to edit the documentation, take out the part about editing your hosts file and add the following:
> After you installed Vagrant, you will need to install an additional plugin that enables vagrant to automagically update your hosts file so you can access your applications more easily.
>```bash
>vagrant plugin install vagrant-hostsupdater
>```
Done.
Actually did that 24 hours ago already but forgot to push to github :)