Add precision for "filter-options" and example for PNG export

[bohwaz]

The --filter-options parameter is plural, so I tried for a long time to get it to take multiple options, eg. comma-separated etc. but it does not.

It should be named "filter-option" really, but in the meantime having it documented properly will help people.

[regebro]

Thanks. I'll also add a --filter-option alias.

[regebro]

Done