Allow installer's defaults to be overridden via the command line

[dscho]

When Git for Windows is installed as a 3rd-party component, it may make sense to override default options, such as "Add Git to the PATH". Let's support that via additional command-line options.

The best way is most likely to add a call similar to ExpandConstant('{param:Choose'+Key) to the ReplayChoice function. A little different, though, because parameters cannot contain spaces.