Allow loading virtual module defined by an async function

[jacquev6]

I think changing load(id: string) to async load(id: string) on line 63 and adding a line module = await module between lines 69 and 70 would allow loading virtual modules defined by async functions. And I think it would not break anything else.

Sorry for not opening a PR; I'm in a hurry :) Thanks for this very useful plugin!