make helm-execute-action-at-once-if-one and helm-quit-if-no-candidate configurable

Most time when I issue helm-imenu, I don't need to think where my cursor is. I just type some characters and jump there. With helm-execute-action-at-once-if-one and helm-quit-if-no-candidate set to t, I have no chance to type characters at a lot of locations. So make them configurable should provide more extensibility.