Check for full match of command name

I ran into an issue with vim-abolish which has a command named `:S`. `exists(':S')` is non-zero because there are other commands that only start with an S, so vim-plug doesn't create the command and my setting `Plug 'tpope/vim-abolish', {'on': 'S'}` does not work. The check if the command exists should check the return value of `exists()` to make sure the full command name exists instead of just a command or multiple commands starting with the same string.