Added MATE.

Hi,

I'm a member of the MATE desktop team, Arch Linux TU and package maintainer for MATE on Arch Linux. I recently added MATE to `[community]`.

I really love what you are doing with Antergos and would really love to see MATE feature in Antergos. This pull request adds MATE to your installer, I hope I've hit everything that needs adding.

Regards, Martin.