Add bup get; see the documentation for further information

Add bup get; see the documentation for further information

WARNING: this is a new EXPERIMENTAL command that can (intentionally)
modify your data in destructive ways.  Treat with caution.

Thanks to Karl Kiniger <karl.kiniger@med.ge.com> for helping track
down various bugs in earlier versions, and for noting that we might
want --verbose to be a little more effusive.  And thanks to Patryck
Rouleau <prouleau72@gmail.com> for suggesting improvements to the
documentation.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>