Make {init,reset}_color_scheme function public

Also rename it from init_color_scheme() to reset_color_scheme(),
which fits better.
Add ":highlight clear" to reset to builtin colors

This balances change in resetting color scheme to default one.
Useful for building color schemes that are partially based on the
builtin one.
Add empty lines to color_scheme.h

For better readability.
Merge branch 'add-highlight-clear'

Add ":highlight clear" to reset to builtin colors

This balances change in resetting color scheme to default one.
Useful for building color schemes that are partially based on the
builtin one.