Refactored _flash.scss mixin for Bitters to allow for customizable text darkening amounts on text and links

_flash.scss

/* replaces /bitters/mixins/_flash.scss */
@mixin flash(
    $color, 
    $darken-text-amount: 50,
    $darken-link-text-amount: 60,
    $darken-link-text-hover-amount: 70
  ) {
  background: $color;
  color: darken($color, $darken-text-amount);
  font-weight: bold;
  margin-bottom: $base-line-height / 2;
  padding: $base-line-height / 2;

  a {
    color: darken($color, $darken-link-text-amount);

    &:hover {
      color: darken($color, $darken-link-text-hover-amount);
    }
  }
}