LaTeX reader:  Don't add spurious ", " to citation suffixes.

This is added when needed in Text.Pandoc.Biblio anyway.
Biblio:  Tweaks to improve default behavior.

* A suffix beginning with a digit gets 'p' inserted before it
  before passing to citeproc-hs, so that bare numbers are treated
  as page numbers by default.
* A suffix not beginning with punctuation has a space added at
  the beginning (rather than a comma and space, as was done before).
* This adding occurs not just in author-in-text citations, but in
  all citations.

The result of these changes (and the last commit) is that
`\citep[23]{item1}` in LaTeX will be interpreted properly,
with '23' treated as a locator of type 'page'.