Add few handy methods to VRadioButtonGroup

[kivan-mih]

Hi Matti, first I want to thank you for Viritin, it's super cool, I'm using it more than 6 years now, it's very handy!

I want to request few methods for Viritin's VRadioButtonGroup, which follow its spirit:

• withLabel
• withItems
• withItemLabelGenerator

Just look at the screenshot and you will understand why I'm asking :)

I'm using flow-viritin 1.4.1