Add new colorschemes without background

[romainl]

Some of the legacy colorschemes specified a ctermbg for Normal while others didn't.

In order to support users who prefer/have to work with a transparent background, we should consider adding a few high-quality "transparent" colorschemes, possibly supporting both bg=dark and bg=light the same way default or retrobox do.

Design constraints:

• focus on color terminals rather than GUI
• reduced palette

[romainl]

A transparent colorscheme could target a specific region of the spectrum. Say your terminal emulator has a dark blue background and you could choose a Vim colorscheme designed to work against a blue-is background. That colorscheme would possibly avoid {dark,light}blue and capitalize on other more contrasted colors like yellows and purples.

I will try my hand at that idea.