Option to add HTML line breaks to multiline text

[harryw]

This change adds a 'multiline' argument to json2html.convert. When enabled, line breaks are replaced with HTML <br/> tags, preserving them in the rendered HTML. This argument is False by default, retaining backwards-compatibility,

This is similar in intent to #19, but uses explicit line breaks rather than <pre> tags, which can lead to a bunch of style changes like monospace font usage.