Add support for JSON based configuration file

[anandsudhir]

Currently, the headless scanner extension expects an XML based configuration file and this works ok. But, making the XML is cumbersome and also requires that most of the request URLs be wrapped in CDATA blocks.

IMHO, this would be much simpler, cleaner and much more concise in JSON than in XML, Also, the configuration files for Burp itself seem to be JSON based. So, this move will make it sort of "comply" with the Burp way of doing things.

[anandsudhir]

For backward compatibility, support for XML based config must also be maintained.