allow a processing instruction to appear multiple times

My use case is to use a processing instruction to group common elements
that will appear multiple times.

<config>
    <extension-module>org.jboss.as.messaging</extension-module>
    <subsystem xmlns="urn:jboss:domain:messaging:3.0">
        <hornetq-server name="live">
            <?LIVE_CONFIG?>
            <?COMMON_CONFIG?>
        </hornetq-server>
        <hornetq-server name="backup">
            <?BACKUP_CONFIG?>
            <?COMMON_CONFIG?>
        </hornetq-server>
    </subsystem>
    ...
</config>

It is not possible currently as the COMMON_CONFIG can only appear once
in the XML.
Using a set of processing instructions allow to reuse the same
processing instructions at different places in the XML.
Merge pull request #13 from jmesnil/multiple_processing_instructions

allow a processing instruction to appear multiple times