This commit introduces a new --oset-metadata key-value-list option,
allowing the user to specify output metadata when encoding
(eg. --oset-metadata=title="Hello",comment="World").
A second option --oremove-metadata is added to exclude existing metadata
from the output file (assuming --ocopy-metadata is enabled).
Not all output formats support all tags, but luckily libavcodec
simply discards unsupported keys.
--copy-metadata describes the result of the option better, (copying metadata
from the source file to the output file). Marks the old --no-ometadata
OPT_REMOVED with a suggestion for the new --no-ocopy-metadata.
