shr.el (shr-tag-color-check): Convert colors to hexadecimal with shr-color->hexadecimal.

shr.el (shr-tag-color-check): Convert colors to hexadecimal with shr-color->hexadecimal.
shr-color.el (shr-color->hexadecimal): Add converting functions for RGB() or HSL() color representation.
shr.el (shr-tag-font): Add.
 (shr-tag-color-check): New function to get better colors.
 (shr-tag-insert-color-overlay): Factorize code between tag-font and tag-span.
shr-color.el: New file.
color-lab.el: New file.