Added -json-flat

NOTE: Patch is against current master. I will update it if other pulls are merged first.

Outputs the JSON in a flattened state. This is very useful for grepping the output, and is a precursor to allowing mapped CSV output (e.g. for SSLPulse).

Snippet:
 >"endpoints.0.grade": "A"
 "endpoints.0.hasWarnings": false
 "endpoints.0.ipAddress": "384.257.412.631"
 "endpoints.0.isExceptional": false
 "endpoints.0.progress": 100
 "endpoints.0.serverName": "blahblahblah.amazonaws.com"
 "endpoints.0.statusMessage": "Ready"
 "engineVersion": "1.11.8"
 "host": "testing.yoursite.com"
 "isPublic": false
 "port": 443
 "protocol": "HTTP"
 "status": "READY"