Add Mixin for HDTV's

Thanks so much for creating this project! ...It's something that I have wanted to do for a while, but never got around to.

One of the things I found missing was a way to target televisions. The code that I added is built on the media query I currently use to target HDTV's. Being that 4K television is coming down the line, I figured that adding a `standard` argument would be a good idea.