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.
@aprescott Are you sure that you can amend a pull request? I did actually amend and force push (mind you, after closing) and the previous pull request still displays the old code. To the best of my knowledge this is because pull requests are tied to commits and not branches (although I could be wrong).

Regarding the use of quotes; I'm happy to change if this how the rest of the project is quotes.

However, in this case the chose was very intentional, as single quotes and double quotes are semantically different (i.e. this string includes no escape sequences so Ruby does not need to parse the string contents).