Allow values for the default date input

[joelanman]

If you don't pass items to govukDateInput, it renders the default Day, Month and Year that most people need. However you commonly need to set the values, at which point you have to set all the items individually.

This PR adds a values option similar to other components that allows you to pass an array of values. In usage this looks like:

{{ govukDateInput({
  id: "passport-issued",
  namePrefix: "passport-issued",
  values: [data['passport-issued-day'],
           data['passport-issued-month'],
           data['passport-issued-year']],
  fieldset: {
    legend: {
      text: "When was your passport issued?",
      isPageHeading: true,
      classes: "govuk-fieldset__legend--l"
    }
  },
  hint: {
    text: "For example, 27 3 2007"
  }
}) }}

The only thing still missing would be to have error states, which could be tricky. You could have a similar classes option, but then you'd have to set govuk-input--width too, at which point the code is pretty much as verbose as setting items individually. However for prototyping I don't think you would always need error states.

[joelanman]

thinking some more, if the main value here is for prototyping, it might be better to look into an option to automatically set all values based on names in the kit (no need to manually set values at all), I think @36degrees did an investigation into that a while back

Update:

Here is the investigation:

alphagov/govuk-prototype-kit#1316