Feature request: fallback for less capable terminals

[lifepillar]

I see that the remade colorschemes support millions of colors and 256-color palettes. Are you planning for a fallback for less capable terminals? For instance, when t_Co is 0, Shine looks like this:

By explicitly supporting a fallback, it might look a bit better (this is using _bw.colortemplate from Colortemplate, but I am not necessarily recommending using that):

By "explicitly supporting a fallback", I mean that there could be a b/w color scheme, which other color schemes could source when they detect that there are too few colors. The b/w color scheme could support a wide range of terminals, e.g., with millions or 256 colors it could use shades of gray, with 16 (resp., 8) colors it could use only ANSI 0,7,8,15 (resp., 0 and 7), and without colors it would only define term entries. This would give a somewhat consistent appearance across all color schemes and across all environments (sure, ANSI colors are arbitrary, but usually 0, 7, 8, and 15 are the darkest/lightest). Then, all other color schemes should only define true colors and xterm colors.

[romainl]

@lifepillar, could you take a look at #26?