Alerta notifier implementation

[Difrex]

See http://alerta.io for more information about it.

"consul-alerts/config/notifiers/alerta/" prefix added

Config options:

• enabled bool Enable/Disable notifier
• url string Url to Alerta API
• token string API token with write access
• cluster-name string The name of the cluster
• domain string You domain. Usable for link templating
• link string Templateable link to smth. Example: https://hashiui.{{ .Notifier.ClusterName }}.{{ .Notifier.Domain }}/consul/{{ .Notifier.ClusterName }}/services/{{ .Msg.Service }}
• origin string
• type string Default: consul-alerts

[fusiondog]

Please fix branch conflicts for further review.