Allow configuring whether `wasm-opt` is required or not

[fitzgen]

In some situations it makes sense to only run wasm-opt if it is available, in others it makes sense to require that it is present and make it a fatal error when it is missing. We should have a flag in the config options to control this behavior.

[xtuc]

Ok, what should be the configuration?

Maybe something like:

new RustPlugin({
    wasmopt: {
        required: true
    }
})

[fitzgen]

Looks perfect!