txt: text message to send to Slack. If a character vector of length > 1 is passed in, they will be combined and separated by newlines.
channel: Channel, private group, or IM channel to send message to. Can be an encoded ID, or a name. See the chat.postMessage endpoint documentation for details.
username: what user should the bot be named as (chr).
icon_emoji: what emoji to use (chr) "" will mean use the default.
thread_ts: Provide another message's ts value to make this message a reply. Avoid using a reply's ts value; use its parent instead.
reply_broadcast: Used in conjunction with thread_ts and indicates whether reply should be made visible to everyone in the channel or conversation. Defaults to FALSE.
...: other arguments passed to the Slack API chat.postMessage call