Add a way to set codec specific payload descriptor flags to Payloader

[at-wat]

For example,

rtp/codecs/vp8_packet.go

Lines 67 to 72 in 3ea3501

	type VP8Packet struct {
	// Required Header
	X uint8 /* extended controlbits present */
	N uint8 /* (non-reference frame) when set to 1 this frame can be discarded */
	S uint8 /* start of VP8 partition */
	PID uint8 /* partition index */

N flag can be set to indicate the frame can be discarded on packet loss. (The value for each frame is given by the encoder.)