Group verbs in help text

If many verbs (tens or hundreds) are implemented, it's very difficult to find a verb. So it would be useful to be able to group them in the help text, perhaps by sections that can be defined as properties of the VerbAttribute.

Example:

```csharp
[Verb("my-verb1", HelpText = "MyVerb1 help", HelpSection = "Section 1")]
class MyVerb1Options
{ ... }

[Verb("my-verb2", HelpText = "MyVerb2 help", HelpSection = "Section 2")]
class MyVerb2Options
{ ... }
```

The output in the console should then group verbs based on their own section.