Add commandline arguments --skip-period-id, --skip-period-duration

[emarsden]

It would be useful to have commandline arguments that allow the user to request that certain Periods in a multi-Period manifest are not downloaded. For example, --skip-period-id could allow a Period with a specific @id attribute (which is printed when fetching with "-v" and "--simulate") to be ignored, and --skip-period-duration could skip all periods whose duration is less than the specified argument.

This functionality could be useful for fetching content which has been enriched with server-side ad insertion (SSAI) or dynamic ad insertion (DAI).