This commit adds support for automatically executing the `wasm-opt`
binary from the [Binaryen project][binaryen]. By default `wasm-pack`
will now, in release and profiling modes, execute `wasm-opt -O` over the
final binary. The goal here is to enable optimizations that further
reduce binary size or improve runtime. In the long run it's expected
that `wasm-opt`'s optimizations may mostly make their way into LLVM, but
it's empirically true today that `wasm-opt` plus LLVM is the best
combination for size and speed today.

A configuration section for `wasm-opt` has been added as [previously
proposed][fitzgen], namely:

```toml
[package.metadata.wasm-pack.profile.release]
wasm-opt = ['-Os']
```

The `wasm-opt` binary is downloaded from Binaryen's [own
releases](https://github.com/webassembly/binaryen/releases). They're
available for the same platforms that we download predownloaded binaries
for `wasm-bindgen` on. We'll also opportunistically use `wasm-opt` in
`PATH` if it's available. If we're untable to run `wasm-opt`, though, a
warning diagnostic is printed informing such.

Closes rustwasm#159

[binaryen]: https://github.com/webassembly/binaryen
[fitzgen]: rustwasm#159 (comment)