Add ability to set saveAssets/saveArtifacts/advanced config

[addyosmani]

Lighthouse currently doesn't provide a way to override the flags object when consuming this API via https://github.com/GoogleChrome/lighthouse/blob/master/lighthouse-cli/bin.ts#L201. Instead, it lets you specify config but the flags object is just consumed from whatever is configured at a CLI level.

Short term we can either PR in a fix for this upstream or host our own copy of bin.js that does allow you to customize the flags object. That would allow our defaultOptions to be fully applied (right now I'm pretty sure only the url and configPath parts are respected.

[addyosmani]

This is now fixed locally.