Support updating a single common AssemblyInfo.cs

Initial work to support issue #224.

This adds support for specifying the filename as: `/updateassemblyinfo ..\src\CommonAssemblyInfo.cs` or `/updateassemblyinfo "C:\src\CommonAssemblyInfo.cs"`.
Why not just `return Directory.GetFiles(workingDirectory, args.UpdateAssemblyInfoFileName ?? "AssemblyInfo.cs", SearchOption.AllDirectories);`