Add option break_at_newlines adapted from GitHub Flavored Markdown

This is the same feature as has already been provided here:

https://github.com/bobtfish/text-multimarkdown/pull/17
https://github.com/bobtfish/text-markdown/issues/15

That implementation is based on the actual GFM Ruby code (which is a hack and is known to be very buggy: https://github.com/github/github-flavored-markdown). It was actually very trivial to add this feature to Text::Markdown.

Although this would probably be more appropriate as a Text::MultiMarkdown feature, looking at the code, it is much saner to implement inside Text::Markdown (parent class).