Allow content in footer and at the bottom of a 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` https://github.com/alphagov/govuk_template/blob/master/source/views/layouts/govuk_template.html.erb