Add basic filestack webhook.

closes #1542

Generalize event.

Add doc.

Update default config.

Add filestack to the list of plugins.

Check that video conversion event returns 400.

Update the readme.

Update the changelog.

plugins/inputs/webhooks/filestack/filestack_webhooks_events.go

@@ -0,0 +1,21 @@
+package filestack
+
+import "strconv"
+
+type FilestackEvent struct {
+	Action    string `json:"action"`
+	TimeStamp int64  `json:"timestamp"`
+	Id        int    `json:"id"`
+}
+
+func (fe *FilestackEvent) Tags() map[string]string {
+	return map[string]string{
+		"action": fe.Action,
+	}
+}
+
+func (fe *FilestackEvent) Fields() map[string]interface{} {
+	return map[string]interface{}{
+		"id": strconv.Itoa(fe.Id),
+	}
+}