Give custom form validators a this context relative to the Vue component

[nahanil]

Could be useful if the validity of one form field is dependent on the state of another.

Some out-of-context nonsense example:

formData: {
  foo: 'foo',
  bar: 'bar',
},

formRules: {
  bar: {
    custom: function(input) {
      return input === 'bar' && this.foo !== 'bar';
    }
  }
}