Support format via NormalizeWhitespace

I'm interested in calling `NormalizeWhitespace` on all C# and VB source files in a folder as a processing tool between `git difftool` and opening up two source trees in Beyond Compare. One scenario that would benefit from this is reviewing semantic changes in this view: https://github.com/icsharpcode/ILSpy/compare/4b865c27...3c6961a9. By calling `NormalizeWhitespace` on the left and right trees of the comparison, all insignificant whitespace differences (including those that span multiple lines) would be ignored by the diff tool.

I propose adding the option `--normalize` for this feature.