Added <details> and <summary> tags to list of HTML elements with block content model (fixes #155)

Note that the <summary> element was using the span content model
prior to this change but the block content model is better
suited (both are allowed by the HTML5 spec).
Line breaks are now recognized in GFM parser when hard_wrap is off (fixes #152)
False method used for detecting string scanner position (fixes #158)

Since the StringScanner#pos= method sets the byte position,
we need to use StringScanner#pos (not #charpos) to get the
correct position for later use.
Fixed incorrect omission of escaped hash sign at the end of atx headers (fixes #164)