Add --require-free-formats

[davidhedlund]

--prefer-free-formats flag exists but not --require-free-formats. Other suggested labels for such flag:

--only-free-formats - #6018 (comment)
--force-free-formats - #6018 (comment)

[phihag]

You can pass in either a list of formats or just -f webm/ogg. I do not see a pressing need for an explicit switch, but you are welcome to contribute a patch that adds one.