Support HTML5 details tag-like behaviour on Firefox and IE

[astratto]

The HTML5 details tag is not supported on Firefox and IE.

We can add an option emulate_details_tag that would inject in the full page a custom Javascript that emulates the folding behaviour of the details tag.

If the option is set, that Javascript is then included in the page.
If format = :partial_html is provided, the Javascript is shipped as well on a separate file.

It's then up to the user to decide whether she wants a fully js-free page or broader compatibility.

[astratto]

Firefox's bug seems to be closed and support for the details tag will be probably available in Firefox 47 (~ June 2016).

Note that as far as I can tell, it's not possible to style the disclosure triangle yet.

Tested on the Nightly Build and collapsing/expanding works as expected though: