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"`.
Yeah, this could work, I also did the `Path.GetFullPath` to handle both relative and absolute paths...
Though it also might be useful to provide a relative root directory, e.g. update all assemblyinfo.cs under this specific dir (and not in the entire repository)