Add support for copying XML from an external file that matches an XPath expression

[GoogleCodeExporter]

I am making heavy use of your plugin to manage my JBoss configuration across 
several environments.  Currently I can insert the contents from one XML file 
into another, optionally including or excluding the root element while doing 
this:

<file ignore-root="true">dir/file</file>

However, instead of just ignoring the root, it would be very useful to allow 
for another XPath expression to select the element or elements to copy.  For 
the time being I believe I can use the xml-maven-plugin to transform my 
original XML document so that I can insert it into my final document.

Original issue reported on code.google.com by gmljacob...@gmail.com on 5 Feb 2015 at 5:32