Provide fallback lookup for partials.

Previously, a file named `blamo/snork/_blah-zorz` would not be able to be looked up. This is problematic since Ember looks up partials with the last part of the name prefixed with an underscore. This is then normalized by our custom `normalize` function before lookup (into `blamo/snork/-blah-zorz'), thus causing a miss on our filename.

A deprecation notice will be logged explaining the issue (and how to correct it).

Closes #22.