Support asynchronous transformers

[egoist]

Feature request

What problem does this feature solve?

Some transformers like remark.js.org allows asynchronous plugins and expose an asynchronous API.

How should this be implemented in your opinion?

Make api.pages.createPage and api.pages.normalizePage return a Promise.

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[egoist]

Turns out remark (which uses unified) has an synchronous API https://github.com/unifiedjs/unified#api (if you don't use async plugins)

cc @limitlessloop