Add preference to show outline automatically on document load

This is a feature that I have missed ever since I started using pdf.js. I've lost count of how many times that I've opened the sidebar and clicked on "Show Outline", just to see that the PDF didn't provide an outline.

This PR implements a, currently hidden, boolean preference in about:config 
`PREF_PREFIX.ifAvailableShowOutlineOnLoad`. When this is set to `true` by the user, the outline will be automatically shown on load, but __*only if*__ it exists.