Grammar refinements, SwiftDoc support, snippets and menu organization

I had intended to hold this back until I had a chance to use it in earnest, but as other people are working on this bundle, I'm sending the PR now, to avoid duplicated effort and unnecessary conflicts.

I've tried to clean up the language grammar a bit more and update it for some new Swift syntax, and added syntax colouring for inline documentation as described in http://nshipster.com/swift-documentation/.

I also updated some snippets and added some of my own, and divided the menu into sub-menus.