Horizontal rules

* added more test cases for `<hr />`,
* added new method `addHorizontalRule()`,
* removed unused lines of code,
* finished implementation so all tests are green.

ping @aquilax @bognix @RafLeszczynski @wladekb 
Hmm wouldn't it be easier to remove all the single spaces in the row and check for 3-5 sequential `*` or `-` ?