Support for "fsc" and "fsi" in the .NET CLI

[cartermp]

Global commands will be a new feature in the .NET Core CLI. This will allow us to install fsc and fsi as commands you can invoke from the command line.

[cartermp]

#3994

[cartermp]

Due to security considerations we will not be using global tools. Instead, we will have reserved commands distributed with the .NET CLI for fsc and fsi.