add diff command to summarize the differences between two cached artifacts

[kevin-hanselman]

Usage:

dud diff <checksum_a> <checksum_b>

or

dud diff <path_to_cached_artifact_a> <path_to_cached_artifact_b>

If the cached artifacts are directory manifests, the directory artifacts are recursively loaded and a diff of the full structure is displayed. (cmp.Diff could be used to accomplish this.)

If the cached artifacts are NOT directory manifests, the location of the first difference is displayed (e.g., "first difference detected at byte X")

If the cached artifacts are a directory manifest and a binary file, display as much.

[kevin-hanselman]

See the discussion in #171