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.

从 MacVim 的 tooltip 或 balloon 实现入手，追踪当前如何渲染插件提供的内容。检查提议的 WebKit 边界，以及应如何在富 HTML 和纯文本之间进行选择。当插件可以在 MacVim 中请求富 tooltip 内容，同时现有的纯文本 tooltip 仍受支持时，即视为完成；不指定任何文件或测试。