Add option to generate it as variables-css

[javierartero]

Generate an option in the cli to generate css variables in a :root {}.

This file must be CSS to be used from any processor.

@function svgpack-imagotype($color: #ef1625) {
  $color: str-replace(inspect($color), "#", "%23"); //fix and replace hexcolor
  @return url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 440 90'%3E%3Cg fill='"+$color+"' fill-rule='evenodd'%3E%3Cpath ...");
}

we need something like this:

:root {
  --svgpack-imagotype: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 440 90'%3E%3Cg fill='%23ef1625' fill-rule='evenodd'%3E%3Cpath ...");
  --others-vars:  url("...");
  }