Improve node programmatic API

[patrickhulce]

Brought over from discussion in #1721 (comment).

The example for how to use lighthouse programmatically is quite daunting for someone just getting started. Splitting out the chrome launcher (#1176) would take us pretty far, but also making basic usage possible without having to require specific files deep within the directory structure would be great.

[gauntface]

I ended up writing a small wrapper for downloading and starting chrome before using the lighthouse node module:

https://github.com/gauntface/gf-site/blob/v3/test/utils/lighthouse-wrapper.js

More than happy to move it out to a node module if it's useful.

[patrickhulce]

Another wrinkle as noted in #1744, the lighthouse results cannot be directly saved as the artifacts have a circular structure

[brendankenny]

I believe this is mostly complete?

• chrome-launcher split to own package
• don't need to require deep files

However, dealing with report/deleting artifacts is still kind of a pain. That part could be merged into the GAR issue.