Proposed update for Vim color schemes

[Lifepillar]

In an effort to move forward with https://github.com/vim/vim/issues/1665, and in particular to address Bram’s comment there, I have “refactored” all Vim color schemes so that they have a similar structure and they pass Vim’s validation (`$VIMRUNTIME/colors/tools/check_colors.vim`). Other than that, the color schemes remain visually unmodified (modulo any mistake on my part). In particular, I have not added new highlight groups (in a couple of cases, I have removed some because they were filetype-specific or plugin-specific).

The color schemes are available for review at https://github.com/lifepillar/vim8-colorschemes.

I am available to make further global changes, if desired. But, to avoid any misunderstanding, I am not offering myself to maintain all those color schemes ;)

Enjoy,
Life.