Implement reset-citation-positions class on headings.

When the `reset-citation-positions` class is added to
a top-level heading, `--citeproc` will reset position
information at that point in the document. This is needed
in order to ensure that the first citation in a chapter
to a work that has been cited in a previous chapter will
not be in abbreviated form.

Requires a dependency on a development version of citeproc.

test/command/reset-citation-positions.md

@@ -0,0 +1,41 @@
+```
+% pandoc --citeproc -t plain --csl command/chicago-fullnote-bibliography.csl
+---
+suppress-bibliography: true
+references:
+- id: foo
+  name: John doe
+  title: A Book
+  type: book
+  publisher: Oxford University Press
+  issued: 2010
+...
+
+# Chapter one
+
+Blah [@foo, p. 7].
+
+Blah [@foo, p. 8].
+
+# Chapter two {.reset-citation-positions}
+
+Blah [@foo, p. 57].
+^D
+Chapter one
+
+Blah.[1]
+
+Blah.[2]
+
+Chapter two
+
+Blah.[3]
+
+[1] A Book (Oxford University Press, 2010), 7.
+
+[2] A Book, 8.
+
+[3] A Book (Oxford University Press, 2010), 57.
+
+```
+