Added return statement to add methods to use them as chain method.

[efeerdogru]

This change is about using add methods as chain methods.

Instead of using like this;

var cmd1 = new Command("cmd1", "Test 1 command.");
cmd1.AddOption(new Option("-o"));

var cmd2 = new Command("cmd2", "Test 2 command.")
{
    cmd1
};

This would be much simplier.

var cmd2 = new Command("cmd2", "Test 2 command.")
{
    new Command("cmd1", "Test 1 command.").AddOption(new Option("-o"))
};

Thanks.

[KalleOlaviNiemitalo]

Since #1989, the options and arguments can be added using C# initialiser list syntax:

var cmd2 = new Command("cmd2", "Test 2 command.")
{
    new Command("cmd1", "Test 1 command.")
    {
        new Option("-o"),
    },
};

Because "Builder types are the only types in BCL that are allowed to use fluent APIs (returning reference to this to allow for calls chaining)" according to #2057 (comment), I don't believe Add will be changed to return other than void.