Add an XML pretty printer database function

Add an XML pretty printer database function

Andrew Dunstan was kind enough to share an XML pretty printer function
for PostgreSQL, and it sure is handy when you're debugging things like
in-database unapi and MARCXML.

Note that it expects honest-to-goodness XML data type input, so for
biblio.record_entry.marc columns you'll need to cast it from TEXT.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Signed-off-by: Michael Peters <mrpeters@library.in.gov>