Add icon_emoji and username options to Slack settings

[jyoung15]

Proposal:
Restore feature parity with Kapacitor Slack event handler by including username and icon-emoji options (https://docs.influxdata.com/kapacitor/v1.6/event_handlers/slack/)

Current behavior:
These options are not available (https://docs.influxdata.com/flux/v0.x/stdlib/slack/message/).

Desired behavior:
username and icon-emoji should be available Slack settings. Possibly consider adding icon_url as well.

Alternatives considered:
Use http.post() and handle alerting externally from Influx.

Use case:
The same Slack webhook URL may be used for multiple alerting purposes. Being able to customize the name and icon for each type of alert helps to easily distinguish the different alert types.