Support scaladoc for subprojects

[dwalend]

Putting site.includeScaladoc() in the top build file wasn't enough to get the scaladocs for the whole project included in site.

Including sbt-site and the site.includeScaladoc() setting in subprojects gets the scaladoc into the zip file. However, that's not useful for putting together a zip file for the whole site.

Please add something to include subprojects' scaladocs via the top-level package-site command. I suspect each subproject should be able to include or exclude itself from the site, with everything included by default.

Thanks,

Dave

[metasim]

Note to contributors: Need to create a scripted test for this scenario and see if it can be done with a combination of the new autoplugin-based configuration and custom Config definitions.

[jonas]

@dwalend Did you consider using https://github.com/sbt/sbt-unidoc?

On a related note, we should add a test for using sbt-unidoc and maybe submit an to update its example. This is also a good use case for testing #60