Add support for querying multiple autodoc indexes at the same time.

[samheutmaker]

Currently Autodoc can only query a package in which it has been directly installed. It would like to support querying dependencies and peer packages that have been distributed with an Autodoc index.

Dependency packages fairly easy. For example, in my autodoc.config.json, I could specify that I want Autodoc into node_modules for packages that have an .autodoc folder and include them when query. This allows for composability of documentation across the dependency graph of any given project.

It's slightly more tough for peer packages. You would need some way to define peers and then have Autodoc pull in their .autodoc indexes from somewhere external.

[slavakurilyak]

Supporting multiple Autodoc indexes would be a useful feature! A potential solution for querying peer packages or multiple Autodoc indexes could be a cloud-based Autodoc database for a fee.