add extension to support one space indent and refactoring on some test cases

Sometimes I think one space indent will be more convenient than 4 spaces.

Actually many other implementation are allowed one space indentation.
It was a [corner case](http://johnmacfarlane.net/babelmark2/faq.html#what-are-some-big-questions-that-the-markdown-spec-does-not-answer) between implementations and the spec.

I think it will be nice if blackfriday support this option?