Add [overlay] configuration section (experimental):

This configuration section uses the new BasicConfig interface that supports
key-value pairs in the section. Some exposition is added to the example cfg
file. The new settings for overlay are related to the Hub and Spoke feature
which is currently in development. Production servers should not set
these configuration options, they are clearly marked experimental in the
example cfg file.
Reviewers: @ximinez @mtrippled 
I think we started talking about this on a prior review. `automatic` seems like a reasonable default until someone typos "nevr". We'd be better off with an explicit check for "automatic" and throw an exception if there's no match.