OPML support (and ability to add custom attributes)

[sjehuda]

OPML support would be useful.
Adding custom attributes would be good too:

• Custom filter rules (See Liferea).
• Custom tags (See Slixfeed).

def export_to_opml(jid, filename, results):
    print(jid, filename, results)
    function_name = sys._getframe().f_code.co_name
    logger.debug('{} jid: {} filename: {}'
                .format(function_name, jid, filename))
    root = ETR.Element("opml")
    root.set("version", "1.0")
    head = ETR.SubElement(root, "head")
    ETR.SubElement(head, "title").text = "{}".format(jid)
    ETR.SubElement(head, "description").text = (
        "Set of subscriptions exported by Slixfeed")
    ETR.SubElement(head, "generator").text = "Slixfeed"
    ETR.SubElement(head, "urlPublic").text = (
        "https://gitgud.io/sjehuda/slixfeed")
    time_stamp = dt.current_time()
    ETR.SubElement(head, "dateCreated").text = time_stamp
    ETR.SubElement(head, "dateModified").text = time_stamp
    body = ETR.SubElement(root, "body")
    for result in results:
        outline = ETR.SubElement(body, "outline")
        outline.set("text", result[1])
        outline.set("xmlUrl", result[2])
        # outline.set("type", result[2])
    tree = ETR.ElementTree(root)
    tree.write(filename)

https://gitgud.io/sjehuda/slixfeed/-/raw/master/slixfeed/action.py?ref_type=heads