convert should read the config, and respect its output option

[Fil]

#770 defaults --output to . It would be nicer to default to docs/ (or the root if specified in the config), so users can start preview/build faster without having to figure out this option.

[mbostock]

Retitled and redesigned: I think it should take the --config option (which defaults to observablehq.config.{ts,js} in the current working directory) and then respect the output option in the config, which defaults to docs. So if you have a different output set, --output will default to that. And maybe we don’t bother reading the config if you specify --output explicitly.