Allow customizing message for Slack notifications

[nawatts]

slack_notifications uses the process name from sys.argv in notifications.

https://github.com/broadinstitute/gnomad_methods/blob/0b8e1007aacabcb81e4093d9e315ac6f7c92651b/gnomad/utils/slack.py#L156-L172

However, someone may want notifications about multiple blocks in the same script. There should be a way to distinguish those notifications.