Support Telegram MarkdownV2 parse_mode

[kva1966]

This is related to the Telegram provider's notify(... parse_mode='html'|'markdown') parameter, defined as:

  parse_mode:
    enum:
    - markdown
    - html
    title: Send Markdown or HTML, if you want Telegram apps to show bold, italic,
      fixed-width text or inline URLs in your bot's message.
    type: string

in https://notifiers.readthedocs.io/en/latest/providers/telegram.html

markdown is now considered legacy, and superseded by MarkdownV2, see: https://core.telegram.org/bots/api#markdownv2-style

The following code:

    telegram = get_notifier('telegram')
    telegram.notify(
        token='<token>',
        chat_id='<chat id>',
        parse_mode='MarkdownV2',
        raise_on_errors=True,
        message='Foo',

Results in:

notifiers.exceptions.BadArguments: Error with sent data: 'MarkdownV2' is not one of ['markdown', 'html']

Happy to take a look and create a PR. Guidance welcome, too.

Thanks for the awesomely useful project.