Initial code, copied over from mi_xmlrpc_ng, with output changed to JSON.

The input is statically set to the "which" command.
libmicrohttpd5 is no longer in Debian/testing
mi_json: simplify datasets

Also rewrote the string handling code.
Modified httpd to support `application/json` for our purposes.
mi_json documentation
Merge pull request #119 from shimaore/mi_json

mi_json module (new module)