Added `--metadata/-M` option.

This is like `--variable/-V`, but actually adds to metadata, not
just variables.
Restore --bibliography, --csl, --citation-abbreviations.

These are now implemented as:

--bibliography FILE => --metadata bibliography=FILE --filter pandoc-citeproc
--csl FILE          => --metadata csl=FILE
--citation-abbreviations FILE => --metadata csl-abbreviations=FILE