Ability to render `html` (or some rich content) in balloon.

I wanted to be able to control the font that is shown for a particular tooltip balloon in MacVim. The use case is for showing compiler errors in monospaced font (otherwise, they render terribly). Then, I quickly realized that I only want some of the tooltip to be monospace and some to be rendered with standard text font. I also wanted greater control over the content in general, and realized that the best way to do this would be to allow the tooltip content to accept arbitrary HTML and have that be rendered using WebKit. I believe this could be done entirely in MacVim and not concern Vim itself. When writing a plugin that shows tooltips, you would want to check if you're running inside of MacVim to determine if you should show rich HTML content vs plain text.

Comienza en la implementación de tooltips o balloons de MacVim y sigue cómo se renderiza actualmente el contenido proporcionado por los plugins. Comprueba el límite propuesto de WebKit y cómo debe seleccionarse entre HTML enriquecido y texto sin formato. Se considera terminado cuando los plugins pueden solicitar contenido enriquecido para tooltips en MacVim y los tooltips existentes de texto sin formato siguen siendo compatibles; no se mencionan archivos ni pruebas.