Adding More APIs - The `wasm-bindgen` Guide

# Adding Support for More JavaScript Global APIs

As of 2018-09-24 we've added all APIs in the current ECMAScript standard (yay!). To that end you'll hopefully not find a missing API, but if you do please feel free to file an issue!

We currently add new APIs added to ECMAScript that are in TC39 stage 4 to this crate. If there's a new API in stage 4, feel free to file an issue as well!

### Instructions for adding an API

- Find the `wasm-bindgen` issue for the API you'd like to add. If this doesn't exist, feel free to open one! Afterwards be sure to comment on the issue to avoid duplication of work.

- Open the MDN page for the relevant JS API.

- Open `crates/js-sys/src/lib.rs` in your editor; this is the file where we are implementing the bindings.

- Follow the instructions in the top of `crates/js-sys/src/lib.rs` about how to add new bindings.

- Add a test for the new binding to `crates/js-sys/tests/wasm/MyType.rs`

- Run the JS global API bindings tests

- Send a pull request!