Added support for Pandoc-style citations.

[Witiko]

This commit adds support for Pandoc-style citations as discussed in #19.

$ lunamark -Xcitations=biblatex -t latex
Smith says blah [-@smith04].
Smith says blah \autocite*{smith04}.

The LaTeX writer produces either vanilla LaTeX2e, Natbib, or BibLaTeX output;
the Natbib and BibLaTeX output has been modelled after the output of Pandoc.
The shell interface has been extended, so that the user can specify either
-citations, +citations, or citations=[latex|natbib|biblatex].

Non-LaTeX writers echo the original citations back. These are normalized, since
the information about the precise input syntax is lost during tokenization.

Unlike in Pandoc, there is no detection of locator terms. Therefore, an
(implicitly enabled) option of citation_nbsps has been added that replaces
all spacing inside the citation pre- and post-fixes with non-breaking spaces.
Non-breaking spaces were added to writers, so they are available for future use.

The citation syntax is a superset of the syntax understood by Pandoc (1.17.2),
since it recognizes @cite1 [; @cite2; @cite3] as a string of three in-text
citations, whereas Pandoc recognizes the input as an in-text citation followed
by a string of two bracketed citations, the first of which is prefixed with a
semicolon.

[jgm]

Looks good!

[Witiko]

I'm glad you find it to your liking. This concludes my string of pull requests,
since all the additional functionality of witiko/markdown has been backported
to lunamark.