Reference Plugin: Make property sourceFileName configurable

* Add ability to completely disable property placeholder expansion

For **Spring XD**, I have been experimenting with incorporating our existing tool-chain with *AsciiDoc*. The idea is that the build of *Spring XD* checks out the the *Spring XD wiki*, then runs *AsciiDoctor* on the reference guide section and then the resulting *Docbook* files are rendered using the *Gradle Reference Plugin* (PDF and HTML). However 2 changes need to be made:

* The way AsciiDoctor generates the docbook XML, the property expansion/filtering used by the Gradle Reference Plugin causes issues. Thus, I need the ability to disable property expansion altogether, including **index.xml**. I am not too happy with choosing a property **neverExpandPlaceholders** but did not want to break any existing functionality.
* I need a way to make the sourceFileName configurable, right now it expects **index.xml**