Publish as global tool

[nil4]

In #18 it was suggested to make dotnet-transform-xdt available as a global tool.

This issue tracks making that happen once .NET Core 2.1 is released and the global tool requirements stabilize and are documented (interim docs available here and here).

[nil4]

With PR #25, preview versions of a CLI global tool and a .NET Standard library are published (but currently unlisted) on NuGet. The tool can be installed with:

dotnet tool install --global dotnet-xdt --version 2.1.0-preview.2

# and then run
dotnet xdt --help

[nil4]

Version 2.1.0-rc.1 published to NuGet:

• dotnet-xdt as a global CLI tool for .NET Core 2.1 and later
• DotNet.Xdt as a .NET Standard library

The "classic" dotnet-transform-xdt 2.0 remains available as a project-level tool for all .NET Core versions.