add shortcut key for nexting a video

Adds a feature where a user can press the 'k' key on the keyboard and btv will go to next the video. Addresses [issue #7](https://github.com/gopperman/breedtv/issues/7). 

I wasn't able to test this code because I don't have a btv wordpress database to test it with. I tried it by copying it into the console on my browser and I got it to work there. I'm not totally confident in the placement of the code in the theme though. Anyway, I'd recommend testing it before merging the code into master.

:bread: :tv: