Add XInclude support

I add support for XInclude which may be defined via "options.xinclude = true".
Merge branch 'patch-1' of https://github.com/dellermann/grunt-xsltproc into dellermann-patch-1

* 'patch-1' of https://github.com/dellermann/grunt-xsltproc:
  Add XInclude support
Add test for xinclude option

  * Re-organise test fixtures directory
  * Test output using xinclude option
  * Update README
Version bump - v0.3.0