Allow GitVersion.exe to run build scripts/exes

[JakeGinnivan]

One thing I liked about GitHubFlowVersion was the ability to do this:

GitHubFlowVersion /proj MyBuild.proj /projArgs "/p:Configuration=Debug"

This would execute the build script and make all the variables available as environmental variables. It also had a /exec flag which did the same thing, but executed anything.

This allowed build scripts to be run locally and have the same variables available as would be available if you ran GitVersion on the build server, then your build script as the second step.

Thoughts @andreasohlund @SimonCropp @nulltoken

[andreasohlund]

I like it

[robdmoore]

+100 I'd like this feature before switching to gitversion. Also, if you have this then potentially you also need the /UpdateAssemblyInfo switch too?

[JakeGinnivan]

Created #111 for the /UpdateAssemblyInfo switch