Combined commands

[anaisbetts]

This PR adds a feature to ReactiveCommand to create a ReactiveCommand that invokes several sub-commands, and whose CanExecute reflects whether all of the child commands can execute. Here's how it works:

var cmd1 = new ReactiveCommand();
var cmd2 = new ReactiveCommand();

cmd1.Subscribe(_ => Console.WriteLine("Command One ran!"));
cmd2.Subscribe(_ => Console.WriteLine("Command Two ran!"));

var newCommand = ReactiveCommand.CreateCombined(cmd1, cmd2, cmd3);

newCommand.Execute(null);

>>> Command One ran!
>>> Command Two ran!

[anaisbetts]

/cc @haacked

[haacked]

Holy shit this is so little code! 👍 Does it pass all the tests we have in GHfW? Great stuff!