Outline destination enhancement (backwards-compatible code, tests and documentation)

Currently, only simple :FIT destinations are allowed for outline destinations.

With this pull request it is allowed to use an Array instead of an Integer for the `:destination` key. When an array is used, it must be a correct destination, for example generated by one of the `#dest_*` methods of `Prawn::Core::Destination`.

Why? The outline normally contains links to headings and there may be more than one heading on a page and it is nice if the PDF viewer jumps to the correct location instead of the top of the page.

I needed this for a kramdown-to-pdf converter (I'm currently patching the outline object directly) and thought this might be useful to others.