Add post_thread

[llrs]

This PR makes it easier to post threads as requested on #172

For the moment I took the fastest approach and only works with text (what I need). The language and other parameters of the thread must be the same and the thread cannot begin as a reply to another toot (To avoid facilitating spam).
I took the liberty to (invisibly) return the ids of the messages posted.

I am not sure of the tests, I've written them but I have not recorded the cassette (I've read the guide but I found confusing if I first need to record the test and then replace it by the fake token token or what...).
Let me know if documentation and tests are enough

[schochastics]

looks good to me. I let @chainsawriot be the judge of the testing

[chainsawriot]

@llrs Thank you for the PR. At the moment, I can see two options

1. Really provide a test with a unique cassette name and the cassette file(s)
2. Remove the test for now; ATM consider the feature to be experimental; we(@schochastics and I)'ll come back to add tests when we have the bandwidth to do so.

[schochastics]

I am fine if the test is removed for now. Thanks @chainsawriot for the input

[llrs]

I removed the test. Thanks for the feedback!
Now to tweak the bot(s) to use this new feature 🚀

[schochastics]

Thanks for implementing this!

[chainsawriot]

Does this person entry look good to you @llrs ? I would like to add you as an author.

person("Lluís", "Revilla Sancho", , "[email protected]", role = c("aut"),
        comment = c(ORCID = "0000-0001-9747-2570"))

[llrs]

@chainsawriot Yes, this is what I usually use. Thanks!