If set, adds a UTCTiming tag in the manifest.

This is part of the recommendations listed in the "Guidelines for
Implementations: DASH-IF Interoperability Points" [1][2]
Section 4.7 describes means for the Availability Time Synchronization.

A usable default is "https://time.akamai.com/?iso"

[1] http://dashif.org/guidelines/
[2] http://dashif.org/wp-content/uploads/2016/12/DASH-IF-IOP-v4.0-clean.pdf
    (current version as of writing)
Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Martin Storsjö <martin@martin.st>