Use more idiomatic receiver name.
Improve debug renderer to wrap around a real renderer.

This is needed because the blackfriday parser behavior depends on the
renderer output. So depending on the renderer implementation used,
there will be difference in what renderer interface methods are called
and their parameters. See russross/blackfriday#106.
Add support for line breaks.

"When you do want to insert a line break using Markdown, you end a line
with two or more spaces, then type return."
Fixes shurcooL/atom-markdown-format#8.
This works contingent on russross/blackfriday#107 being merged. Until
then, it will not work and tests will fail.