Aggregate finders

[sagikazarmark]

Currently there is no way to specify pairs of paths and names as a search criteria (ie. all names are looked up in all paths which may not be desirable).

One potential solution is eliminating the difference between paths and names.

Another solution is creating a separate finder implementation that aggregates different finders (maybe running them in parallel).

This may not result in the best performance, but the primary purpose of this library is to support configuration loading use cases where performance is rarely a concern.

Introducing alternate implementations comes with a new challenge though: we need a finder interface in this library. The name Finder is already taken though.

[sagikazarmark]

I decided to tackle this in spf13/viper#1849 instead.