Helper utility to access the active renderer for a given extension.

def renderer(name):
    """
    Helper utility to access the active renderer for a given extension.
    """
    try:
        if name not in Store.renderers:
            extension(name)
        return Store.renderers[name]
    except ImportError:
        msg = ('Could not find a {name!r} renderer, available renderers are: {available}.')
        available = ', '.join(repr(k) for k in Store.renderers)
        raise ImportError(msg.format(name=name, available=available))