`frontmatter.detect_format()` use `handler.detect()`

[NGenetzky]

The following function doesn't properly ask the handlers if they can "detect" and instead utilizes the key of the dictionary which is assigned to handler.FM_BOUNDARY.

# global handlers
handlers = {
    Handler.FM_BOUNDARY: Handler() 
    for Handler in [YAMLHandler, JSONHandler, TOMLHandler]
    if Handler is not None
}


def detect_format(text, handlers):
    """
    Figure out which handler to use, based on metadata.
    Returns a handler instance or None.

    ``text`` should be unicode text about to be parsed.

    ``handlers`` is a dictionary where keys are opening delimiters 
    and values are handler instances.
    """
    for pattern, handler in handlers.items():
        if pattern.match(text):
            return handler

    # nothing matched, give nothing back
    return None