/// Return whether $version is valid SemVer
/// @param {String} $version
/// @return {Boolean}
/// @example scss
///   $valid: valid('1.2.3');
///   -> true
///   $not-valid: valid('a.b.c');
///   -> false
@function valid($version) {
  $version: ($version or '') + '';
  @return str-index($version, '.') != null and
    type-of(major($version)) == 'number' and
    type-of(minor($version)) == 'number' and
    type-of(patch($version)) == 'number';
}