Minimal example of custom language plugin

While working on a custom language plugin, I found myself wishing for a truly minimal example of a custom language plugin as a sandbox for figuring out the intricacies of the IntelliJ API. I wrote one, and I thought it might be useful to others working along similar lines.

https://github.com/georgewfraser/hellolang

It implements a very simple language with expressions like:

let x = "Hello world!" in x

So far I've implemented syntax highlighting, error and info annotations, local reference resolution, in-place rename refactoring, code formatting, and test fixtures. Pull requests with additional platform features are encouraged.