Add toolbar URL parameter

This PR adds the `toolbar` URL parameter to pdf.js, see the specification: http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_open_parameters_v9.pdf#page=7.

Fixes #2784 and [bug 844350](https://bugzilla.mozilla.org/show_bug.cgi?id=844350).

__Work in progress, do not merge!__