C#/.NET language bindings

It would be nice to have C# language bindings for Jsonnet. C# is a great language for web sites and services, and is a very popular language in enterprise. A simple wrapper library could call a libjsonnet DLL via [PInvoke](https://msdn.microsoft.com/en-us/library/55d3thsc.aspx).

.NET assemblies are cross-language compatible, so the C# bindings could be used in Visual Basic and F#. .NET Core and Mono support would be nice to have since they are cross-platform, but neither have anywhere near the userbase of the standard .NET framework. So this Issue is scoped to only the standard framework.

Dependent on Visual Studio/VC++ support (#300). PInvoke works with any DLL so it's possible to do this using MinGW DLLs, but that makes building more complicated.