Allow citation positions to restart

[stakats]

Some styles (e.g. dissertation formatting guidelines) require citations to "reset" in each chapter. Thus even a source cited in earlier chapters will receive a full citation when first cited in a subsequent chapter. At present the only way to produce a document conforming to this style is to create separate files for each chapter. Two possibilities come to mind:

1. Zotero could detect section breaks and restart positions in each section, much as Word allows users to restart footnote numbering in new sections.

2. Alternatively, Zotero could insert a reset field into docs after which the positions would restart.