New reporter module for Alerta

Adding a new reporter module largely inspired by the existing
``reportSlack`` to report events in Alerta (https://alerta.io/) via the
HTTP API.

config.yml.example

@@ -129,7 +129,21 @@ reports:
 #        visibility: "++BGPalerter-5-${type}: ${summary}|${prefix}|${description}|${asn}|${earliest}|${latest}|${peers}"
 #        misconfiguration: "++BGPalerter-3-${type}: ${summary}|${asn}|${prefix}|${earliest}|${latest}"
 
-
+#  - file: reportAlerta
+#    channels:
+#      - hijack
+#      - newprefix
+#      - visibility
+#      - path
+#      - misconfiguration
+#    params:
+#      severity:
+#        hijack: 0
+#        newprefix: 2
+#        visibility: 4
+#        path: 7
+#      urls:
+#        default: _YOUR_ALERTA_API_URL_
 
 ############################
 # Notification settings: