Add helper for filling in telephone fields

[frankieroberto]

Following the addition of fill_in_govuk_text_field in #12 we should add a similar fill_in_govuk_telephone_number_field helper (or maybe fill_in_govuk_tel_field?).

As well as checking that type="tel", this could additionally check that:

• autocomplete="tel" (could be opt-in or opt-out as you wouldn't want this if asking for someone else’s phone number)