When creating a Slack receiver, the adv. config. options only allow user to customize an icon and the username.  We should also add textareas for Title and Text which would allow user to fully customize the alert messages sent to Slack.

Example yaml:

 receivers:
  - name: Critical
    slack_configs:
      - channel: '#critical-alerts'
        icon_emoji: ':exclamation:'
        username: dtaylor113
        link_names: false
        title: >-
          {{ .CommonLabels.alertname }} {{ if eq .Status "firing" }}({{
          .Alerts.Firing | len }}{{ end }} {{ .Status }}) 
        text: >-
          {{ range .Alerts -}} *Alert:* {{ .Annotations.title }}{{ if
          .Labels.severity }} - `{{ .Labels.severity }}`{{ end }}

          *Message:* {{ .Annotations.message }}

          *Description:* {{ .Annotations.description }}

          *Details:*
            {{ range .Labels.SortedPairs }} • *{{ .Name }}:* `{{ .Value }}`
            {{ end }}
          {{ end }}
  - name: Warnings
    slack_configs:
      - channel: '#warning-alerts'
        icon_emoji: ':warning:'
        title: >-
          {{ .CommonLabels.alertname }} {{ if eq .Status "firing" }}({{
          .Alerts.Firing | len }}{{ end }} {{ .Status }}) 
        text: >-
          {{ range .Alerts -}} *Alert:* {{ .Annotations.title }}{{ if
          .Labels.severity }} - `{{ .Labels.severity }}`{{ end }}

          *Summary:* {{ .Annotations.message }}

          *Message:* {{ .Annotations.message }}

          *Description:* {{ .Annotations.description }}

          *Details:*
            {{ range .Labels.SortedPairs }} • *{{ .Name }}:* `{{ .Value }}`
            {{ end }}
          {{ end }}