Add download commands for additional bootstrapper files

[gep13]

With the upcoming release of Cake 1.0.0, there are now 3 supported ways of running Cake scripts:

• .NET Tool
• .NET Framework
• .NET Core

Currently, the Visual Studio extension allows you to download Bootsstrappers (.ps1 and .sh) for only .NET Framework.

Commands should be created to support downloading the boostrappers for all the available options.

[gep13]

This has now been extended to include 6 menu items: