Enable toc and cover options

rebase of https://github.com/pdfkit/pdfkit/pull/158 on top of current master, and updates to make the options work.

Original PR notes -

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.