Make it accept "," as separator, instead of only ":". Do this by using
the key-value-list parser. Before this, the option was stored as a
string, with the option parser verifying that the option value as
correct. Now it's stored pre-parsed, although the log levels still
require separate verification and parsing-on-use to some degree (which
is why the msg-level option type doesn't go away).
Because the internal type changes, the client API "native" type also
changes. This could be prevented with some more effort, but I don't
think it's worth it - if MPV_FORMAT_STRING is used, it still works the
same, just with a different separator on read accesses.