Add support for asynchronous processors

[ismay]

I'm in the middle of writing a stylelint-processor for glamorous, but it needs postcss-js to parse the css-in-js, which works async. So I can't return a result synchronously. The docs only show a sync example, so my question is if there is a way to define an async processor.

[davidtheclark]

@ismay: Probably not. Didn't think of that. We can build it in as a new feature — it's a good idea.

[jeddy3]

@ismay Feel free to help contribute this feature. Support for async plugins was recently added, and might be useful as a reference.

[ismay]

Cool, I'll look into it if I have the time!

[jeddy3]

Closing as processors are discouraged in favour of custom syntaxes.