add priority -2 to pushover node.

to close #129

social/pushover/57-pushover.html

@@ -37,7 +37,7 @@
         <input type="text" id="node-input-name" placeholder="Name">
     </div>
     <div class="form-tips" id="node-tip">Tip: Leave title blank to set using <b>msg.topic</b> and <b>msg.priority</b>.<br/>
-    Only supports priorities 2, 1, 0, and -1.</div>
+    Supports priorities 2, 1, 0, -1, and -2.</div>
 </script>
 
 <script type="text/x-red" data-help-name="pushover">

social/pushover/57-pushover.js

@@ -45,7 +45,7 @@ module.exports = function(RED) {
             var pri = this.priority || msg.priority || 0;
             if (isNaN(pri)) {pri=0;}
             if (pri > 2) {pri = 2;}
-            if (pri < -1) {pri = -1;}
+            if (pri < -2) {pri = -2;}
             if (typeof(msg.payload) === 'object') {
                 msg.payload = JSON.stringify(msg.payload);
             }
@@ -73,6 +73,6 @@ module.exports = function(RED) {
         credentials: {
             deviceid: {type:"text"},
             pushkey: {type: "password"}
-        }       
+        }
     });
 }