example:
        {% strip '<br>' %}
            content...
            content...
        {% endstrip %}
    result:
        content...<br>content...
    arg special symbols: {\n} = \n

def strip(parser, arg):
    """
    example:
        {% strip '<br>' %}
            content...
            content...
        {% endstrip %}
    result:
        content...<br>content...
    arg special symbols: {\n} = \n
    """
    nodelist = parser.parse(('endstrip',))
    parser.delete_first_token()
    return StripNode(nodelist, arg.split_contents())