Allow content in footer and bottom of page

* Two new options `footer_top` and `body_end`
* `footer_top` is for custom footers
* `body_end` is for markup that should be right at the bottom of the
page, eg javascript includes
* New options use the same names as govuk_template
Merge pull request #15 from alphagov/more-nested-layouts

Allow content in footer and at the bottom of a page