Close #2588 Add extra content-for hooks

Added extra `content-for` hooks that allow you to inject code at the bottom of the head and body tags from an ember-cli addon.

    * `{{content-for 'head-footer'}}`
    * `{{content-for 'test-head-footer'}}`
    * `{{content-for 'body-footer'}}`
    * `{{content-for 'test-body-footer'}}`

No explicit tests as there we no existing ones for the other hooks that I could see.