Implement minimal JSON Print

The JSON archive uses the RapidJSON::PrettyWriter, which inserts
a bunch of extra whitespace.  This change adds an option to use
the RapidJSON::Writer, which generates a minimum-length string.

Signed-off-by: Erich Keane <erich.keane@verizon.net>
$0.02: Do you need a ```unique_ptr<>``` if the only place it's initialized is the constructor, where an initializer list can be used?