Feature request: Add support for SDP precondition/QoS attributes as defined in RFC 3312 #31

The current SDP grammar rules do no support precondition attributes as defined in RFC 3312, such as:

a=des:qos mandatory local sendrecv
a=curr:qos local none
a=des:qos optional remote sendrecv
a=curr:qos remote none

The generated JSON includes these attributes under invalid, like this:

"invalid": [
	{
		"value": "des:qos mandatory local sendrecv"
	},
	{
		"value": "curr:qos local none"
	},
	{
		"value": "des:qos optional remote sendrecv"
	},
	{
		"value": "curr:qos remote none"
	}
],

These attributes are widely used in telephony, and it would be nice to have them properly supported in a future release.