Extract slack message config to the separate file.

[LaserPhaser]

Configuration file for slack message/emoji must be an as separate file for easy configuring.

[lithammer]

I think this is just a matter of using parser.addini(...) instead of .addoption(...) in pytest_addoption().

[LaserPhaser]

Not quite.
I meant here that it would be a nice feature to be able to use file with
--slack_config='path to file' option.
In slack config file should be possible to configure the slack payload in any way.
For example, some users maybe want to attach screenshots, add some buttons or something else.