Citation changes.

* Citations will work in markdown even if `--biblio` isn't
  specified.  Note:  this may cause unexpected behavior for people
  who use strings of the form `@foo` that are not citations!
* If `--biblio` isn't used, the markdown writer will write markdown
  citations rather than CSL-rendered citations.
* This means, for example, that you can do `pandoc -f latex -t markdown`
  and convert biblatex or natbib citations into pandoc citations.
Merge branch 'citations'
Implement `--default-image-extension` for LaTeX reader.