Resolve issue #119, add hardbreaks option

Turns out this was a really easy feature to implement, so I went ahead and did it.

I call this the "make the GFM crowd happy" feature :wink: 

I applied the replacement where line break characters are normally substituted. If we find that it's putting line breaks in too many places, we can always scale it back.

...note that this does slow down processing a pinch...0.05s on the AsciiDoc User Guide. But that's also because the User Guide hard wraps just about every single line of paragraph text.