fix "expandable links" for idrefs that refer to ids not in current document

[Matt Coarr]

expandable links that reference target ids that are not in the current document are no longer labeled as "[external]".

also, the ids were not being displayed for these "external" idrefs (because the new logic from issue #35 caused the id to be printed to be pulled from the target object, and since that target object was not available, the id was not properly being printed.)

[Matt Coarr]

this is fixed in 2f1416c