Support single-trigger commands constructor

[tommasobertoni]

We should provide the following Command constructors, mainly useful for simple commands definition:

// default, current state
new Command()

// single-trigger command
new Command("TurnOnLight", "turn on the light");

// multi-trigger command
// IEnumerable<string> and params string[]
new Command("TurnOnLight", "turn on the light", "light on", "light on");