Support AssemblyInfo.cs from custom locations

[ImrePyhvel]

Some solution use the practice of having a solution-wide AssemblyInfo.cs file that's included to every contained project. Currently generation logic expects the source file in a conventional location.

Consider adding option to change the source file location -or- if possible with good performance - to automatically discover the file from some alternate locations (ex: from next to solution file).

[ImrePyhvel]

Introducing a configurable MSBuild property named $(AssemblyVersioningBaseVersionFile) which defaults to the conventional location of Properties\AssemblyInfo.cs. Users can override this in their local props file to point to whichever location.

Similarly made configurable where the output file would be written. This can now be controlled by setting property $(AssemblyVersioningOutputFile).

I'm reluctant to add any automation since the default convention is a strong one and I'm not aware of any other conventions with wider usage to warrant extra surfing around during build. Users can set it up once, during install.

[ImrePyhvel]

Released in v0.12.0.