Get all files matching :ext: from :start: directory.

def _all_files_matching_ext(start, ext):
    """Get all files matching :ext: from :start: directory."""
    md_files = []
    for root, _, files in os.walk(start):
        md_files += fnfilter([os.path.join(root, f) for f in files],
                             "*." + ext)

    return md_files