Add TagRule to use value of H1 element as page title and drop the element

New rule H1ToTitleRule to remove the first H1 element of a page (should have only one anyway) and reuse its value as page title. A skin might add such an element later easily again at any location and it helps to generate proper meta-data if the underlaying page format (e.g. Markdown) does not support such a definition of a title.