Automatically execute the selected command

When a command is selected on the list, it appears on the command line but it is not executed. I sometime only want to execute command without having to specify any parameters. 

I've added the ``g:ctrlp_cmdpalette_execute`` to add <CR> after the command. 

I might add another option later to manage an exclude list for the commands that need parameters. Perhaps there is also a way to check if the selected command needs arguments.