Add filtering by message author to /clear (#169)

Closes #164 

This PR adds an optional argument to `/clear` - `author` of type User.

If the user is specified, only messages sent by that user will be
cleared. Simple as that.

src/Messages.Designer.cs

@@ -980,5 +980,21 @@ namespace Octobot {
                 return ResourceManager.GetString("GuildInfoBoostCount", resourceCulture);
             }
         }
+
+        internal static string NoMessagesToClear
+        {
+            get
+            {
+                return ResourceManager.GetString("NoMessagesToClear", resourceCulture);
+            }
+        }
+
+        internal static string MessagesClearedFiltered
+        {
+            get
+            {
+                return ResourceManager.GetString("MessagesClearedFiltered", resourceCulture);
+            }
+        }
     }
 }