Add RuleSeverity for AvoidUsingWMIObject rule

Modified the column width of "Severity" in Invoke-ScriptAnalyzer output
format.

Author: yutingc

Engine/ScriptAnalyzer.format.ps1xml

@@ -13,7 +13,7 @@
           <Label>Rule Name</Label>
         </TableColumnHeader>
         <TableColumnHeader>
-          <Width>10</Width>
+          <Width>12</Width>
           <Label>Severity</Label>
         </TableColumnHeader>
         <TableColumnHeader>

Engine/ScriptAnalyzer.types.ps1xml

@@ -42,7 +42,7 @@
             <Name>DefaultDisplayPropertySet</Name>
             <ReferencedProperties>
               <Name>Name</Name>
-              <Name>CommonName</Name>
+              <Name>Severity</Name>
               <Name>Description</Name>
               <Name>SourceName</Name>
             </ReferencedProperties>

Rules/AvoidUsingWMIObjectCmdlet.cs

@@ -116,6 +116,17 @@ public SourceType GetSourceType()
             return SourceType.Builtin;
         }
 
+
+        /// <summary>
+        /// GetSeverity:Retrieves the severity of the rule: error, warning of information.
+        /// </summary>
+        /// <returns></returns>
+        public RuleSeverity GetSeverity()
+        {
+            return RuleSeverity.Warning;
+        }
+
+
         /// <summary>
         /// GetSourceName: Retrieves the module/assembly name the rule is from.
         /// </summary>