Add support for multiple source directories to the hologram config

- Source now allows either a single value (keeping backwards compatibility)
  or a yaml array of sources.
- This is useful when pulling in a library that comes with Hologram docs
  support built in.
- We considered auto resolving conflicting namespacing for multiple
  libraries, but we weren't sure that was really an essential feature.
  We'll use this feature for a bit, and add it if it seems worth the
  effort.
- We didn't modify the CLI to take multiple sources, if you'd like that
  feature, we're happy to add it.