Adding support for modmails

[Creekie1337]

Is your feature request related to a problem? Please describe.
I'd like to automate some flows within Modmails.

Describe the solution you'd like
The ability to get streams of modmails (The new modmail - not the old)

Describe alternatives you've considered
N/A

Additional context
https://not-an-aardvark.github.io/snoowrap/ModmailConversation.html

[brenapp]

This should be pretty quick to implement. If you want to take the lead, that would be fantastic! The most relevant decision you'll have to make is how to distinguish different modmail conversations (maybe compare the JSON object?)

I've set up the Poll class such that it's very very easy to extend and add new streams

[brenapp]

I've gone ahead and implemented a very basic version of ModMailStream (without any testing 😃) in snoowrap@1.1.2, see if you can stress test in for your purposes.

[Creekie1337]

Hey there, I have updated to the latest version however I am getting the TypeError: ModMailStream is not a constructor error. After looking at your code, I believe you're using the old modmail.

You should be using something like this:
r.getNewModmailConversations({limit: 2}).then(console.log)

[brenapp]

Upgraded snoowrap to the newest version to get new method. The new version should be working correctly, I've tested it successfully on my test servers, but I don't moderate any larger subreddits which would be better test grounds