Footer: separate data from the component

[ppcano]

Implements #56

The component has been extended to support a few cases.

    {{es-footer }}

    {{es-footer tagline="A framework for ambitious web developers"}}

    {{#es-footer as |f|}}
      {{f.info infoLinks=infoLinks}}
      {{f.statement socialLinks=socialLinks}}
      {{f.contributions contributorLinks=contributorLinks}}
    {{/es-footer}}

    {{#es-footer as |f|}}
      {{#f.info}}
        <br/>
        <a>Team</a>
        <br/>
        <a>Contact</a>
      {{/f.info}}
      {{#f.statement}}
        Highly Productive Out of the Box
      {{/f.statement}}
      {{#f.contributions}}
        <div class="contributor">
          <p>Hosted by:</p>
          <a href="https://www.heroku.com/emberjs">
            {{svg-jar "heroku-logo" class="contributor-logo"}}
          </a>
        </div>
      {{/f.contributions}}
    {{/es-footer}}

The addon documentation has been included. I wait for feedback to modify the API and/or improve the documentation.

[mansona]

This seems like a significant improvement from before so I think we should merge it now any improvements we have can be added iteratively 👍