title: WTForm "OR" conditional validator? (Either email or phone)
body: <pre><code>class ContactForm(Form):
    name = StringField('Name',
                       validators=[DataRequired(), Length(max=255)])
    email = StringField('Email',
                        validators=[Optional(), Email(), Length(max=255)])
    phone = StringField('Phone number',
                        validators=[Optional(), NumberRange(min=8, max=14)])
    comment = TextAreaField(u'Comment',
                            validators=[DataRequired()])
  </code></pre>
  
  <p>Is there anyway to specify a validator such that either <code>email</code> or <code>phone</code> is required? </p>