docs: def compile_slides(source):
    '''Compiles the source to a remark.js slideshow.'''
    # if it's a directory, do all md files.
    if os.path.isdir(source):
      for f in os.listdir(source):
        if f.lower().endswith('.md'):
          compile_markdown_file(os.path.join(source, f))
    else:
      compile_markdown_file(source)