[merces]

This adds:

• a new :colorscheme command to set the theme (well, the color scheme :))
• Vim's default-dark and default-light adaptive themes (they use the terminal background and foreground colors in many places).
• Vim's darkblue theme.

New themes:

Vim default (dark background): :colorscheme default
Vim default (light background): :colorscheme default-light
Vim darkblue : :colorscheme darkblue

This are not done the same way as in Vim. There's no :set background=dark for example. That's why default and default-light are actually two different themes.