Allow support for legacy/bot tokens, not just webhooks

[patcon]

Hi @stevenosloan! Thanks so much for this tool!

Might you be willing to accept a PR that allowed it to work not only with webhooks, by using legacy or bot api tokens? This would ideally allow the ability to set ~~username and~~ avatar.

(I'm trying to make a team tool that I would empower any team member to set it up, and I hope to avoid resulting messages confusingly coming from the user who supplied the token.)