Conda package diff subcommand

[analog-cbarber]

It would be useful to be able to compare the difference between packages generated using
this tool vs conda-build.

To do this, you could:

1. Use cph extract to extract both packages to temporary directories
2. Run your favorite directory diff tool
3. Discard the directories

We could add a diff subcommand that does this and allows for saved user config
for how to run the diff tool.

[analog-cbarber]

After extraction the info files should be canonicalized to minimize differences