Handle MultiMarkdown-style citations.

This is a borrowed idea (1, p. 23).

> This is a cited blockquote without a page reference. (2).

And a plain paragraph.

And a code block that is not a citation:

    This is not a citation[p. 5][#Doe:1996].

And a work by Johnson that is not cited, but should be included in bibliography:

And multiple citations. (1, p. 3) (2, p. 10)

How about a citation within a footnote? 1

1. As described previously. (2, p. 12) ↩

Bibliography

[1] John Doe. Some Book. Blog Books, 1996.

[2] James Smith. Some interesting paper. Some Journal, 1(2): 101-123, 2005.

[3] Bill Johnson. Some Book. Blog Books, 1996.