Add XDM support and option to include nvidia modules in initramfs

[porschemad911]

Hi, added support for XDM and an option to include nvidia modules in initramfs (Debian-based distros). I had to modify display manager detection regex for Debian, but should work with other distros. Feel free to change as needed. I am also unsure how to include modules in initramfs for non-Debian distros, so please add if you know how to do this. Lastly I am no python guru (second program I have touched only), so apologies for any bad coding practices.

Closes #61