Fluent API: AddOption should return the Option

Currently we have to write this:

```
var fileOption = new Option(
name: "--file",
description: "The mdp-accelerator file to use.");

rootCommand.AddOption(fileOption);
```

With a minor change to the API, we could write this instead:

```
var fileOption = rootCommand.AddOption(new Option(
name: "--file",
description: "The mdp-accelerator file to use."));
```

This would allow us to define, register, and capture the option all in one line instead of having to use two separate statements. And all it would cost is adding `return option;` at the end of the `AddOption` function.