Proposal: add a paragraph about WebAssembly

[maudnals]

Even though WebAssembly is not tightly bound to JS, it can relief its performance in any environment that has a JS engine (among others).

So: would it make sense to include a mention of WebAssembly? Especially since there's already a paragraph on asm.js, one of Wasm's "ancestors".

Chapter 5 of Async & Performance might be the right place (the asm.js paragraph is also in there):
https://github.com/getify/You-Dont-Know-JS/blob/master/async%20%26%20performance/ch5.md

What do you think?

[maudnals]

Awesome!!