Markdown citations

see http://johnmacfarlane.net/pandoc/README.html#citations

> Citations go inside square brackets and are separated by semicolons. Each
> citation must have a key, composed of ‘@’ + the citation identifier from
> the database, and may optionally have a prefix, a locator, and a suffix.
> The citation key must begin with a letter or _, and may contain
> alphanumerics, _, and internal punctuation characters (:.#$%&-+?<>~/).
> Here are some examples:
>
>     Blah blah [see @doe99, pp. 33-35; also @smith04, ch. 1].
>
>     Blah blah [@doe99, pp. 33-35, 38-39 and *passim*].
>
>     Blah blah [@smith04; @doe99].
>
> A minus sign (-) before the @ will suppress mention of the author in the
> citation. This can be useful when the author is already mentioned in the
> text:
>
>     Smith says blah [-@smith04].
>
> You can also write an in-text citation, as follows:
>
>     @smith04 says blah.
>
>     @smith04 [p. 33] says blah.