allow "@meta=value" form in addition to "@meta value"

Allow metadata to be specified as a single argument. Track-specific
metadata works the same way. The key-value delimiter is "=". "=" may
not appear in a key as this is how ffmpeg splits the metadata
internally, so there shouldn't be any problems.

Why the change?

  % cdimport @{1..8}:title='The Council of Elrond' ...

instead of

  % eval "$(print -- cdimport @{1..8}:title\ '"The Council of Elrond"' ...)"

for example.
normal synopsis formatting

Arguments that you can specify multiple times use "...", not "*".
* version 1.3