Add option to hard break lines via hardbreaks attribute

[GoogleCodeExporter]

In AsciiDoc, you can hard wrap paragraphs of text and they will be combined 
into a single paragraph. While this is expected behavior for some people, to 
others it's not.

Add a feature to AsciiDoc to treat newlines in paragraph content as real line 
breaks.

The next paragraph contains two phrases separated by a single newline character:

Roses are red
Violets are blue

When the following attribute is set:

:hardbreaks:

AsciiDoc would output (when using an HTML backend)

Roses are red<br>
Violets are blue

(naturally, other backends would use the appropriate line break character).

I propose for this substitution to occur where ever the line break character 
(+) is processed.

Original issue reported on code.google.com by dan.j.allen on 27 Jan 2013 at 6:58