adds RegisterCustom function to tag package

[flicaflow]

From the discussion of the unmerged and probably obsolete PR #271 the idea was to Just a RegisterCustom function to the tag package. This PR is exactly that. I needed the TrackingID and TrackingUID tags which are currently not available. Following code enables me to use them now:

var (
	TagTrackingID  = tag.Tag{Group: 0x0062, Element: 0x0020}
	TagTrackingUID = tag.Tag{Group: 0x0062, Element: 0x0021}
)

func init() {
	tag.RegisterCustom(tag.Info{
		Tag:  TagTrackingID,
		VR:   "UT",
		Name: "TrackingID",
		VM:   "1",
	})
	tag.RegisterCustom(tag.Info{
		Tag:  TagTrackingUID,
		VR:   "UI",
		Name: "TrackingUID",
		VM:   "1",
	})
}