Fix bg colour in terminal vim

In terminal vim with 256 colours, colour 0 is often not black (or white) or any predictable colour at all. In solarized for example, it could be a white-shade or the blue-marine bg colour. 

Jellyx.vim was using colour 0 as the background. But in terminal vim, I like to have a transparent background, and set the bg colour with Xresources. I think transparent bg makes more sense because colour 0 could be anything at all. 

gvim should be unaffected. Where I'm using `NONE` for `cterm{bg,fg}`, I'm using `black` for `gui{fg,bg}`. 

Thought you might want to consider it. :+1: