Verbatim blocks should not output <pre> HTML elements when rendered

We define the verbatim block as a way to escape wiki markup so that it is not interpreted, so it's purely a syntactical element. The definition of the <pre> tag is a visual one, text that has already been formatted and should not be reflowed by the browser; thus, it has a semantical meaning that is not implied by the markup. Plus, it adds element when one isn't needed, simply because we just want not to interpret wiki markup in a piece of text.