Add support for fetching subthreads

[rneatherway]

Fixes #46

[mwarkentin]

@rneatherway thanks! Confirmed this worked for me after checking out this branch:

❯ go run ./cmd/gh-slack/ --limit=1000 'https://sentry.slack.com/archives/C06MA1ST3B5/p1709648539357359?thread_ts=1709320542.716369&cid=C06MA1ST3B5'
> **m** at 2024-03-05 09:22 EST
>
> <https://github.com/rneatherway/gh-slack>
> Utility for archiving a slack conversation as markdown
>
> It's a bit weird, but I do it through the github cli :smile:

>
> Find its nicer than manually copying the channel contents


> **j** at 2024-03-05 09:29 EST
>
> neat! copy-pasting has the disadvantage that it does not cover threads, so this looks nice

> **m** at 2024-03-05 09:41 EST
>
> I don't think this pulls in threads either, unfortunately
>
> You can grab threads by using the thread ID

[rneatherway]

Now released as 0.0.23 🎉