Support custom settings

Issue #10 exposes an issue in that we only support a subset of Lighthouse settings. We should support all [settings supported by the NPM module](https://github.com/GoogleChrome/lighthouse/blob/888bd6dc9d927a734a8e20ea8a0248baa5b425ed/typings/externs.d.ts#L82-L119).