New theme simple-16

[Ergus]

Hi:

I just added a new branch with a new theme called simple-16. This is a
very basic theme based in the vim's default color schemes in terminal
implemented only with 16 colors to look exactly the same (or as close as
possible) either in terminal as when using the gui.

This will benefit specially the terminal exclusive users (yes, we
exist); but maybe some of those who like dark themes in gui will find it
attractive too.

I don't use gnus and some other functionalities, so if one those user
want to try the theme and suggest improvements are, of course, very
welcome.

Best,
Ergus