Add macros (discussion, do not merge)

[joelanman]

In order to add auto data storage (#157), we need macros to generate form fields. This is because the code to populate field values with data the user has entered is complex. I don't think users of the kit will find it easy to write that code for each field.

Macros also help us make prototypes accessible, again without our users having to learn that HTML.

I think we should try and keep each macro as simple as possible as an MVP.

This PR is for discussion, for example is macro. the right naming convention? Should allowing users to easily add their own macros be part of this PR?

Example of usage:

<form action="">

  <div class="form-group">

    {{ macro.text('First name') }}   

  </div>

  <div class="form-group">

    {{ macro.text('Last name') }}  

  </div> 

  <div class="form-group">

    <h2 class="heading-medium">
      Role
    </h2>

    {{ macro.radio('Role', 'Teacher') }}

    {{ macro.radio('Role', 'Student') }}

    {{ macro.radio('Role', 'Administrator') }}

  </div> 

  <div class="form-group">

    <h2 class="heading-medium">
      How do you get to school?
    </h2>

    <p>Select all that apply</p>

    {{ macro.checkbox('Bicycle') }}

    {{ macro.checkbox('Bus') }}

    {{ macro.checkbox('Car') }}

    {{ macro.checkbox('Train') }}

  </div> 

</form>

[joelanman]

Previous work here: #99

[timpaul]

Suggest we close this now. We're developing templates as part of GOV.UK Frontend so once the Kit is consuming that they will be available to it's users.

[timpaul]

Closing this now

[joelanman]

Just a note that macros for production might be too complex for the users of the kit, so we may still need our own.