New feature: Add variable processing to XDT


It would be nice to have variable processing in XDT:
1) add an <xdt:Include file="filepath" [xpath="xpath to parent node"] /> tag allowing to include all childs of the parent/root node of the referenced xml file as childs of the current xdt:Include parent node.
2) add an <xdt:Variable name="varname" value="varvalue"/> tag allowing to define the value of a variable.
3) add a text post-processing step remplacing $(varname) by varvalue.

4) Optional but recommended : when executed from Visual Studio, inject standard VS macros $(ProjectName), $(SolutionDir), $(ConfigurationName) ... as XDT variables.

Remark : the text post processing can be applied after transform (on the resulting XML) but should be applied before evaluation of xdt tag and attribute values (Transform, Locator, Import, Include and Variable). Allowing for example to correctly interpret:

<xdt:Import file="Proxy$(ConfigurationName).config"/>