Don't prefix them with --, and allow metatags without content.
Moved --quiet to first command, otherwise it conflicted with the toc option when setting it. Sadly the order in which you set the meta tags is important when using toc, because wkhtmltopdf needs the toc options to be grouped, and after the toc tag.
'global options' 'cover' 'toc options' 'page options'.
This does have a disadvantage: you cannot specify specific page options for toc or cover, and you can't put a cover in the middle of a pdf.
For example, if you'd like to add a page-number to your TOC with a footer, that's currently not possible, because footer is a page option. This could be fixed by parsing the content of the 'toc' parameter, and translate that into other options.