LaTeX reader: citation handling changes.

Previously, a LaTeX citation would always be parsed as a Citation
element, with the raw LaTeX in the [Inline] part.

Now, the LaTeX citation is parsed as a Citation element only if
`--biblio` was specified (i.e. only if there is a nonempty set
of references in readerReferences).  Otherwise it is parsed as
raw LaTeX.

This will make it possible to simplify some things in the markdown
writer.  It also makes the LaTeX reader behave more like the Markdown
reader.