Sidebar dropdown now remembers the article

See #21.

Code review remark:
the final JS string that lands in the HTML file is 
`document.URL.match(/\/[^\/]*$/)`
Seems that Jade processor does some stripping hence in the jade file it's needed to escape the backslashes:
`document.URL.match(/\\\/[^\\\/]*$/)`

This works fine both on usermanual index page (which is `/usermanual/<version>/`) and articles (which are `/usermanual/<version>/foo-bar`).