async compiler

## Feature request

#### What problem does this feature solve?

Returning a promise from the function passed to `vm.config.renderer.code` doesn't work as one would expect.

I wanted to use an async method (`crypto.subtle.digest` in my mermaid plugin:

```js
hook.mounted(function() {
const renderer = vm.config.markdown.renderer
const original = renderer.code || renderer.origin.code

renderer.code = renderer.origin.code = async function(code, lang) {
if (lang === 'mermaid') {
const sha256 = Array.from(new Uint8Array(await window.crypto.subtle.digest('SHA-256', new TextEncoder().encode(code)))).map(b => b.toString(16).padStart(2, '0')).join('')
const mermaidId = `mermaid-svg-${sha256}`
return `${mermaid.render(mermaidId, code)}`
}
else return original.apply(this, arguments)
}
})
```

But all my code fences are now being replaced, in the HTML document, with:

```html
[object Promise]
```

#### What does the proposed API look like?

No change, but the compiler should be promise-aware (so that when the result of calling a function is a promise, it awaits it).

#### How should this be implemented in your opinion?

I'm not sure how the compiler works at the moment.

#### Are you willing to work on this yourself?

If time allows, which is probably negative :-/