Preserve Github Flavor Markdown code block language information

Take two - https://github.com/xijo/reverse_markdown/pull/40

The previous pull request wasn't technically incorrect i.e. Github did/does recognise the highlighting when a space is included before the language identifier. However, looking at the official documentation it would seem that the preferred form is not to include a space (https://help.github.com/articles/github-flavored-markdown/).

NOTE: Compared to my previous pull request I've also swapped to stream style string concatenation, for consistency with the existing code.