Added a Label class to help in displaying static text.

It includes
 - A caching system to save on the amount of calls to `pong::render_text`.
 - Setter and getter methods for text (std::string), text size (std::size_t), and position (pong::math::vector).
 - Rendering function which only calls `pong::render_text` when the cached surface is out of date.
 - Documentation. It's not so great.