Add a helper for filling in email fields

[frankieroberto]

Following the addition of fill_in_govuk_text_field in #12 we should add a similar fill_in_govuk_email_field helper.

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

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