Fix -j/--json config setting

The -j/--json config option for specifying a config file did not work. The problem was that after invoking getopt_long to parse the command line options, the clerk_init() function overwrote the 'json' field of
the clrk_clerk_t object.
This change assigns the argument passed to the -j/--json option to a temporary that is passed to clerk_init() and is used to initialize the 'json' string of the global clrk_clerk_t directly.
Thx for your change!

I would like to stick with the name 'json' instead of 'config' due the file only holds the data and doesn't configure clerk at all.