Proposal: making `render()` async

As we discussed [in the PR over node-sketch-bridge](https://github.com/Lona/node-sketch-bridge/pull/6#issuecomment-575070941), I'm working on writing a "pure" implementation of `makeImageDataFromUrl()`, using `fetch` and `@skpm/fs`.

There is one major problem implementing it in this way though: `fetch` is asynchronous, while the whole rendering operation is synchronous.

Now, I see two ways around this:
1. I keep pieces of the existing implementation that uses `NSData` around, and keep everything sync. Not surprisingly, [NSData documentation](https://developer.apple.com/documentation/foundation/nsdata/1413892-init) has a big fat warning about not using it for network requests, though, because it can potentially hang the whole thing if the network is slow/unresponsive. The pro is that I can keep the existing interface. The con is that, as per current implementation, it can potentially hang Sketch, and the implementation is less uniform.
2. Make `makeImageDataFromUrl` return a Promise and make the whole chain of functions up to `render` async as well. This allows us to use `fetch`, makes the whole thing faster in cases where there are multiple images in a layout, and in general less prone to hang, but it's a breaking change.

@mathieudutour Thoughts?

Begin with makeImageDataFromUrl and render, then read the linked node-sketch-bridge PR and this discussion to determine whether the rendering chain should become Promise-based. Done requires a recorded API decision and agreed implementation scope for the breaking change.

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