Use callbacks to implement async return

[FWeinb]

There is as solution to "return" things from wasm but as you said in the video because everything is async the only way to do it is to use a callback as a second parameter. This PR exposes the function blackfridayFormat(input, cb) where input is a string and cb is a function that takes the resulting string as the first parameter.

[yamalight]

Very cool! Thanks for highlighting this 👍