send_message data slightly outdated being able to control unfurl'ing would be nice

[zaughon]

Looking at the documentation for
https://api.slack.com/methods/chat.postMessage
https://api.slack.com/methods/chat.delete
the current data dict sent for send_message seems a bit out of date. Several things are supposed to be booleans, and some things are not supported by chat.delete (probably not causing issues, then we would've seen it, but still..).

Submitting a merge request that fixes a few of those, and allows you to set unfurl_links, unfurl_media and link_names through msg.extras.