Improve diff target

[russcam]

The diff target is able to diff

• assemblies: two assemblies
• directories: pairs of assemblies
• github: the output of two github commits
• nuget: the lib and its dependencies

The implementation does not currently handle

• Finding the nearest target framework match in dependent nuget packages (this functionality is exposed in Nuget.Frameworks)
• Ability to diff locally compiled current state against a nuget/github
• Diff'ing IL merged NEST assemblies (it does not error, but the output fails to pick up appropriate changes).

Improvements should be made to address these issues.