Fix colorscheme switching problems at template level about vim-colortemplate

[lifepillar]

@tpope's comment is spot on there.

As things stand currently, the only "clean" way to fix the issue of messed up highlighting after color switching is to avoid overriding the highlighting of syntax files in colorschemes entirely.

I see two ways to encourage colorscheme developers to follow such a policy in templates:

1. Raise a warning every time a highlight group different from the "core" ones is defined;

2. Put a conditional block in the default templates, along these lines:

   verbatim
   if get(g:, '@optionprefix_enable_syntax_hi_groups', 0) " Skip this block by default
     " Highlight groups for syntax highlighting are defined here
   endif
   endverbatim

   preceded by a comment explaining why defining syntax highlight groups in a colorscheme is a bad idea. The two are not mutually exclusive, of course.

[lifepillar]

I have added a “Best practices” section to Colortemplate's help file recommending, among the rest, to avoid filetype-specific highlight groups or to define them conditionally.

As far as I can see, this issue not solvable in other ways (except by plugins like colorscheme-switcher—no idea how effective they are, though), not unless something changes in Vim. I will leave this open for the time being, in case someone comes with a better idea.

[lifepillar]

Closing, because this issue has been fixed in Vim (vim/vim#4405).