Spoiler tag

Requires javascript to work.

The quote design is reused, but instead of the `::before` pseudoelement an actual `span` element is used as it is not possible to trigger the click event on a pseudoelement (there are hacky workarounds, but neither of them is really dependable and/or cross browser).

The tag is added to the MarkItUp! editor.

The editor icon is ugly, I should have blurred it to match the other icons.

implements #52