Add ParseTagBlock func

[icholy]

Give users direct access to tag block parsing logic.

Motivation: #111

[aldas]

looking this PR alone - it is OK. but #111 (comment) has valid point. That function returns tagblock but there are no "generic" way to assign that tagBlock to sentence if you decide to parse sentence out of rest of line.

	tag, n, _ := nmea.ParseTagBlock(raw)
	if n > 0 {
		raw = raw[n:]
	}
	if strings.HasPrefix(raw, "$AIVDM") { // fix one specific sentence here
		raw = "!" + raw[1:]
	}
	m, _ := nmea.Parse(raw)
	vdm, ok := m.(nmea.VDMVDO)
	if n > 0 {
		vdm.TagBlock = tag // can assign tag to known type but what if I deal many different sentences?
	}

[kimgr]

I like this patch too, for what it's worth. It would be helpful to me to be able to parse only the tag block.

[icholy]

@aldas I need a review