Add keywords parameters for slack backend

[AntoinePrv]

I added keywords parameters in the slack backend to enable other parameters from the config, such as the username and the image (Fancier than the Slack Api Tester default name).

I mentioned it in the readme and also corrected: if you use the @ symbol the message is send to a Slack user which is no longer supported by Slack Api (only user ids are supported).

Thanks

[coveralls]

Coverage decreased (-0.6%) to 82.77% when pulling b1a821f on AntoinePrv:slack into 9b5e7d2 on dschep:master.

[MACscr]

No luck with this? Having the sender name as "Slack Api Tester" is pretty annoying.

[szpak]

That feature would be also useful for me to be able to use different sender icons, depending on the context (e.g. warning vs alert). @dschep Do you plan to accept it (in one way or another) or definitely not and it is required to use a patched version?

[dschep]

i have no issue merging if the merge conflicts were resolved

[szpak]

@AntoinePrv It has been some time, but maybe you still use that feature and would like to have it merged upstream (by resolving the conflicts)? :-/