Org reader: Fix distinction of images and normal links

Fixed a false assumption about the precedence of (&&) vs (||).
Org reader: Fix parsing of footnotes

Footnotes can consist of multiple blocks and end only at a header or at
the beginning of another footnote.  This fixes the previous behavior,
which restricted notes to a single paragraph.
Move `compactify'DL` from Markdown reader into Shared

The function `compactify'DL`, used to change the final definition item of a
definition list into a `Plain` iff all other items are `Plain`s as well, is
useful in many parsers and hence moved into Text.Pandoc.Shared.
Org reader: Allow for compact definition lists

Use `Text.Pandoc.Shared.compactify'DL` to allow for compact definition
lists.