Removed dependency on citeproc-hs.

Going forward we'll use pandoc-citeproc, as an external filter.

The `--bibliography`, `--csl`, and `--citation-abbreviation` fields
have been removed.  Instead one must include `bibliography`, `csl`,
or `csl-abbrevs` fields in the document's YAML metadata.  The filter
can then be used as follows:

    pandoc --filter pandoc-citeproc

The `Text.Pandoc.Biblio` module has been removed.  Henceforth,
`Text.CSL.Pandoc` from pandoc-citations can be used by library users.

The Markdown and LaTeX readers now longer format bibliographies and
citations.  That must be done using `processCites` or `processCites'`
from Text.CSL.Pandoc.

All bibliography-related fields have been removed from `ReaderOptions`
and `WriterOptions`: `writerBiblioFiles`, `readerReferences`,
`readerCitationStyle`.

API change.