cmd/list: list casks only if one is installed
cmd/list: list casks only if one is installed

vitorgalvao/brew,claui/brew,vitorgalvao/brew,JCount/brew,sjackman/homebrew,claui/brew,MikeMcQuaid/brew,sjackman/homebrew,nandub/brew,EricFromCanada/brew,JCount/brew,JCount/brew,EricFromCanada/brew,JCount/brew,sjackman/homebrew,MikeMcQuaid/brew,EricFromCanada/brew,Homebrew/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,vitorgalvao/brew,claui/brew,nandub/brew,vitorgalvao/brew,Homebrew/brew,Homebrew/brew,sjackman/homebrew,claui/brew,nandub/brew,EricFromCanada/brew