Add another sourcing strategy

See Tarrasch/antigen-hs#11
Add support for plugins in subdirs

This is what Tarrasch/antigen-hs#11 was actually about
Add sourcing strategy: filePathsSourcingStrategy

Also an example using it, and another example showing where it might be
nicer to not use it.
Merge pull request #13 from Tarrasch/support-subdir-and-loading-strategy

Support subdir and loading strategy