Consolidate AssemblyInfo update logic

[asbjornu]

I added a few TODOs in cb4698d for consolidation of the AssemblyInfo update logic in GitVersionTask and GitVersionExe that should be tackled. There's quite a bit of duplicated functionality that leads to diverging support for different features and bugs in the task that might be well tested in the .exe and vice versa. Consolidating all of this into GitVersionCore with accompanying tests would be neat.