Add asdf dd command

[embray]

A quite potentially useful utility I've been meaning to add to the asdf command-line tool is a simple dd command functioning more-or-less like dd. Maybe doesn't need every conceivable feature but in general the ability to extract binary block data directly from the ASDF file is useful.

The same code can then easily be used as the underpinning for an asdf explode command for creating an exploded ASDF file.