Allow transform files to be manually specified as a command line parameter

[LHCGreg]

It's common to have many build configurations that are identical other than the configuration transforms used. For example, one may have a project with QA, RC, and production configurations which use the same build settings but have different config transforms. These extra configurations bleed into other projects in the solution. Those configurations could be reduced to a single Release configuration if the config transform file could be specified at build time.

Something like

msbuild /p:Configuration=Release;ConfigTransformFile=app.QA.config