Dumping stream data of/or specific objects

Adds a command line option -ds to dump the raw body data of a stream object after filter-decoding it.

Also includes a general (optional) command line argument that selects a specific object to print (as JSON) instead of the whole file.

The object IDs are expected in the form n.g or n g as a single argument, so as to support future extension to selecting multiple objects (could be useful to output a content stream that is split over multiple stream objects, for example).