[Slack sender] The channel argument is redundant with the webhook_url argument

[martinp7]

First of all thanks for the work !

It seems the channel argument is redundant with the webhook_url argument since the creation of a Slack webhook URL requires a channel : the information of the channel is included in the webhook

TODO :

• remove the argument channel, to avoid breaking change it can set to optional argument instead
• update the slack section of the README.md

@VictorSanh if you are OK with this, I can do the pull request