Improvement: DotNetCore buildserver shutdown

Could you please add command to terminate build server in planned .net core 2.1
Based on announcement
https://blogs.msdn.microsoft.com/dotnet/2018/04/11/announcing-net-core-2-1-preview-2/

`dotnet buildserver shutdown`

> This command can be used in CI scripts in order to terminate worker processes after builds are completed.

The issue does not mention specific files, tests, or entry points. Start by locating Cake's command-line handling and existing build-server commands, then compare the requested `dotnet buildserver shutdown` behavior with the linked .NET Core announcement. Done means the command can terminate build-server worker processes after a build and is covered by appropriate tests.

Written by the indexing model from the issue text.