lispy.el (lispy-newline-and-indent-plain): add and bind to "RET"

This is useful when enabling lispy in the minibuffer for
`eval-expression', e.g.

    (defun conditionally-enable-lispy ()
      (when (eq this-command 'eval-expression)
        (lispy-mode 1)
        (local-set-key "β" 'helm-lisp-completion-at-point)))

    (add-hook 'minibuffer-setup-hook 'conditionally-enable-lispy)