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.

Begin in MacVim's tooltip or balloon implementation and trace how plugin-provided content is currently rendered. Check the proposed WebKit boundary and how rich HTML versus plain text should be selected. Done means plugins can request rich tooltip content in MacVim while existing plain-text tooltips remain supported; no files or tests are named.