Fix #60: Don't crash on /blob/ with non-file
Fix #59: Show download link instead of "Binary data not shown" and "Large file not shown"
Performance improvement for too-large text files

Don't unnecessarily pygmentize() text that isn't going to be displayed.
Fix #56: Enable 'toc' and 'extra' markdown renderer extensions

Thanks to @ar4s for the initial patch!
Fix #57: Replace 'timesince' with 3rd party implementation