Add support for async processor

[Plawn]

I want to use async function in a processor to fetch an external resource but I can't seem to use an async function.

Having an async processor would make extending the base processor way easier

Thanks !

[ggrossetie]

I agree but Asciidoctor.js is transpiled from Ruby and basically it's not possible (very hard) to support async functions. You can take a look at https://github.com/ggrossetie/unxhr in order to make synchronous HTTP requests in Node.