Add support for passing parameters to TextTransform.exe command line.

This PR enhances the package by adding the ability to pass arbitrary parameters  to the `TextTransform.exe` command line. (See the docs for the -a argument `TextTransform.exe` to at http://msdn.microsoft.com/en-us/library/bb126245.aspx) Parameters are gathered from items of type `TextTransformParameter`, where the item Identity metadata specifies the parameter name and special Value metadata specifies the parameter value. This approach was inspired by the `T4ParameterValues` item documented at http://msdn.microsoft.com/en-us/library/ee847423.aspx.