Add govuk_password_field helper

[jsugarman]

What

add a govuk_password_helper to the form builder

Why

For me this makes sense to have alongside the govuk_email_field
when you are implementing, for example, authentication - for consistency
if nothing else.

# example for typical `devise` sign in page
= form_for(resource, as: resource_name, url: session_path(resource_name)) do |f|
  = f.govuk_error_summary
  = f.govuk_email_field(:email, label: { text: 'Email' }, autofocus: true, autocomplete: 'email')
  = f.govuk_password_field(:password, label: { text: 'Password' }, autocomplete: 'current-password')

[peteryates]

This looks great Joel, thank you for the contribution - extra kudos for going the extra mile and updating the guide.

My only tiny nit pick is that you've not configured your name (via git config user.name) so the commits default to your github handle. I fixed that but it might be worth setting up (the command takes a --global flag if you want to do it for all projects)

[peteryates]

Manually merged in d8a7071

[jsugarman]

Brill, and thanks for the hint @peteryates . will look into. Have not done much git config TBH