add package headers

Adds basic package headers so package can be used with melpa 
`lexical-binding` should be set at first line.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Using-Lexical-Binding.html

First line should be like this.
```lisp
;;; nm.el --- Nevermore an experimental email interface for Notmuch -*- lexical-binding: t -*-
```