Explicitly set C# version

[sandervrijders]

After cloning the repo and opening it in Visual Studio 2017, IntelliSense complains about missing language features. This is since by default the last major version of C# is used (7.0) and certain features are 7.1+ (such as async Main). I suggest setting the LangVersion in the csproj files to latest:

<LangVersion>latest</LangVersion>

[mjsabby]

Feel free to send a PR. You can use Directory.Build.props and set LangVersion there.

[tannergooding]

I put up a PR that does this and configures infrastructure for the repo in general: #60

[tannergooding]

Should be resolved now.