feat: add saas.alerting.Xmatters (#712)

Adding xMatters node to alerting.

Co-authored-by: jared.curtis <jared.curtis@everbridge.com>

diagrams/saas/alerting.py

@@ -20,4 +20,8 @@ class Pushover(_Alerting):
     _icon = "pushover.png"
 
 
+class Xmatters(_Alerting):
+    _icon = "xmatters.png"
+
+
 # Aliases

docs/nodes/saas.md

@@ -10,6 +10,7 @@ Node classes list of saas provider.
 - **diagrams.saas.alerting.Newrelic**
 - **diagrams.saas.alerting.Opsgenie**
 - **diagrams.saas.alerting.Pushover**
+- **diagrams.saas.alerting.Xmatters**
 
 ## saas.analytics