Allow pass extra optional arguments to Slack API

[rennerocha]

When sending a Slack message we don't have an easy way to provide extra arguments to postMessage method (mrkdwn or thread_ts for example). It would be good to be able to provide extra arguments.

My use case is that I have an already created thread on a monitoring Slack channel and I want that Spidermon notifications goes to a specific thread instead of going as a new message on the main channel.

[andersonberg]

I can handle this if anyone is assigned

[rennerocha]

I can handle this if anyone is assigned

It would be great!

[rennerocha]

This issue is related to https://github.com/scrapinghub/spidermon/issues/313 . We need to allow these extra arguments using the new Slack library.