[PATCH] Add pretty-printed XML output option

Hi,

This small patch introduces a XML pretty print function. It basically
takes advantage of the indentation feature of xmlDocDumpFormatMemory
from libxml2 to format XML strings.

postgres=# SELECT xmlpretty('<foo id="x"><bar id="y"><var
id="z">42</var></bar></foo>');
        xmlpretty
--------------------------
 <foo id="x">            +
   <bar id="y">          +
     <var id="z">42</var>+
   </bar>                +
 </foo>                  +

(1 row)

The patch also contains regression tests and documentation.

Feedback is very welcome!

Jim