Auto-download wasm-opt if not present

[thedodd]

Now that the work in #115 has landed (thanks @dnaka91), we should be clear to start working on the auto-download of wasm-opt.

• if a user has explicitly set wasm-opt level to 0 (off), then we should suppress any attempt to fetch wasm-opt (let's give the user some control so that they don't end up in a frustrating situation).
• let's update the default to 3 or 4 (@dnaka91 probably has a good recommendation here) so that it will be invoked by default.
• have a look at Leverage wasm-pack to handle wasm-bindgen installs & other awesomeness #5. It appears as though there is already some good work on this front which we could leverage.

[thedodd]

Done.