Feature Request: text-light & text-dark as transparent

[PascalPixel]

1. text-dark and text-light are classes used to make sure the text differentiates when we apply a special background.
2. As text is usually already black, and we have a text-white class for pure white, the current behavior doesn't add functionality
3. If text-light and text-dark were instead based on rgba(255,255,255,.5) and rgba(0,0,0,.5), they would better represent their name; lightness and darkness

Current:

Suggested:

[mdo]

PR would be great! :)