Support for MPD Anchors

Created by: LloydW93

Dash.js already has support for "#s=n", where n is a start offset in seconds. However, ISO23009-1 Appendix C specifies four MPD anchors:

Key	Value	Semantics
t	Time or time range in the same format as defined in Media Fragments URI 1.0	Time since the beginning of the period indicated by the period parameter. If the t parameter is not present, its default value is 0.
period	String	Period@id. If period parameter is not present, the default value is the ID of the Period with the earliest PeriodStart.
as	string	Value of a single AdaptationSet@id
track	string	Value of a single AdaptationSet@group

Dash.js should add support for at least #t, as this is something that current works if you pass an media url to a native media tag in a browser (https://www.w3.org/TR/media-frags/).