Cleaned whitespace
Any valid destination can now be an outline destination

Before only simple references to pages could be done. Now, instead
of using an Integer for the :destination key, it is possible to
use an Array with a custom destination (and such an array can
easily be generated with the various #dest_* methods).
Merge pull request #552 from gettalong/outline-destination-enhancement

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