Add creating .NET Core Global Tool package

[philippgille]

.NET Core 2.1 introduces ".NET Core Global Tools", inspired by npm. This is a great feature and we should also support this.

• .NET Core 2.1 Preview Announcement Blog Post
• Official sample
• Blog post by Nate McMaster

This doesn't replace Chocolatey or AppImage, because it requires the .NET Core runtime to be installed, which is not the case with our Chocolatey or AppImage packages.