Add support for new_chat_members

[Kyle2142]

The singular field has been deprecated for a long time (https://core.telegram.org/bots/api-changelog#may-18-2017) and has been removed from the documentation.
Please add support for the plural version.
Currently I do

@bot.handle('new_chat_member')
def example(chat, new_chat_member):
  new_chat_members = chat.message['new_chat_members']
  for new_chat_member in new_chat_members:
    foobar

but if telegram removes the field, all of us will no longer be able to receive these updates.
I think I can just make the decorator argument plural, but I am unsure.

I imagine this is just adding the plural version to the message types constant, but again I am unsure