Separate prefix options for directories and filenames in partial references

I'm introducing this as a possible resolution for issue #9.

This changeset adds options support for `prefix_dir` and `prefix_file`, which clarifies prefix configuration for partial references that use a path. The original `prefix` option continues to be available, and has the same behavior as before. The included documentation recommends `prefix_dir` and `prefix_file` but still mentions `prefix`.

Also included are new unit tests for a partial reference using a path but no configured partial directory or prefix (which works in the current version), `prefix` using a path (to test the old weird behavior), `prefix_file` by itself, `prefix_dir` by itself, and `prefix_file`/`prefix_dir` combined.

@5thWall, @franz-josef-kaiser: Feel free to take a look and let me know if you see any issues or problems with this approach.