Add the ability to configure the eyaml file extension

This adds the ability to configure the eyaml file extension to the work done by @terrimonster.
I didn't like hardcoding it either, but the ciphertext will be returned from hiera lookups if the eyaml file extension is set to yaml and the yaml precedes eyaml in the backends array.  There's probably a better solution, but I didn't think of it at the time.