MB-11820: Create json source for cbtransfer

It will load json files in zip format or in a directory that contains
json files, or a simple json file.

Change-Id: I913091b3a11516fac434a1837d7e5604837eaf27
Reviewed-on: http://review.couchbase.org/40657
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>