Add logic to CommandService.Execute to handle basic permissions checks

src/Discord.Net.Commands/CommandService.cs

@@ -208,7 +208,14 @@ namespace Discord.Commands
             if (!searchResult.IsSuccess)
                 return searchResult;
 
-            var commands = searchResult.Commands;
+            // TODO: this logic is for users who don't manually search/execute: should we keep it?
+            
+            IReadOnlyList<Command> commands = searchResult.Commands
+                .Where(x => x.CanExecute(message)).ToImmutableArray();
+
+            if (commands.Count == 0 && searchResult.Commands.Count > 0)
+                return ParseResult.FromError(CommandError.InvalidPermissions, "Invalid permissions");
+
             for (int i = commands.Count - 1; i >= 0; i--)
             {
                 var parseResult = await commands[i].Parse(message, searchResult);