Support IcedCoffeeScript keywords

[joewhite]

Related to #99: When I'm editing an IcedCoffeeScript file (.iced extension), it would be nice if await and defer were highlighted as keywords (or in some way had a different color from ordinary identifiers).

In plain old CoffeeScript (.coffee), await and defer are just ordinary identifiers. So it might make sense (if it's easy to do so -- I know very little about authoring TextMate bundles) to only highlight the extra keywords if the filename is .iced. Then again, IcedCoffeeScript can run plain old .coffee files too (and runs them as IcedCoffeeScript, await/defer and all), so it's debatable whether it should be extension-sensitive.