Allow changes of the xslt configuration, used by HTML -> FO transformation

In Jira Cloud, the value of the issue.description field is stored in HTML.

The following velocity example transforms raw HTML markup to FO so it can appear correctly in the resulted PDF document.

$pdfRenderer.asRendered($issue, 'description', $issue.description)

Problem: minor discrepancies in formatting may occur between the export output and the UI.