Add option to honor line breaks

[mojavelinux]

AsciiDoc requires a plus sign offset by a space at the end of a line to create a hard break (in HTML, a <br>).

Example:

Roses are red +
Violets are blue

We should add a hardbreaks attribute that would tell Asciidoctor to break wrapped lines without having to append the plus character. This feature aligns with an extension of GFM. We could lobby to have this included in AsciiDoc as well.

Example:

Roses are red
Violets are blue

This would apply wherever regular paragraph text is used (probably just in the paragraph block).

[mojavelinux]

I call this the "make the GFM crowd happy" feature 😉

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.

[mojavelinux]

Here's the upstream issue in AsciiDoc: http://code.google.com/p/asciidoc/issues/detail?id=16