Adds onValidation prop to GiftedForm

[danielweinmann]

Now you can pass an onValidation prop to GiftedForm and it will run on every change of every widget.

If you don't pass it it does not validate, so we don't have worry about performance. Here is an example that solves @ScandyMark's #6:

export default class Form extends Component {
  constructor(props, context) {
    super(props, context)
    this.state = {
      isValid: false,
    }
  }

  handleValidation(validation) {
    if (this.state.isValid !== validation.isValid)
      this.setState({ isValid: validation.isValid })
  }

  render() {
    return (
      <GiftedForm
        formName="yourForm"
        validators={yourValidatorsHere}
        onValidation={this.handleValidation.bind(this)}
      >
        <GiftedForm.TextInputWidget name='fullName' />
        <GiftedForm.SubmitWidget
          title='Sign up'
          isDisabled={!this.state.isValid}
        />
      </GiftedForm>
    )
  }  
}

[FaridSafi]

Hello, thank you for your work Daniel, i'm currently traveling so I can not merge easily. I added you as collaborator, do you have a NPM account?

[danielweinmann]

Thanks! If you don't mind I'll still send PRs just for you to check the code before I merge it...is it ok with you?

Just created npm account: also danielweinmann ;)

[FaridSafi]

Sure, I published 0.0.3

[peterknudsen]

The onValidation doesn't work with SelectWidget. I have a validator on the SelectWidget itself, but the _validate method in WidgetMixin iterates over the individual OptionWidgets and doesn't fire because there are no validators on any of the options.