Allow NPM publishing

Hi there!

I was glad your module was there to help me export 100+ issues from a GitHub project  :)

As a token of appreciation, as the manual installation of dependencies is rather unusual with Node project thanks to [NPM](https://npmjs.org/), I wrote a `package.json`, which allows dependency management and publishing to the NPM public registry. This is what you'll find in this changeset.

I did not publish `github-issues-export` yet, in case you'd prefer to [publish](https://npmjs.org/doc/publish.html) it yourself. I can of course do it, otherwise.

I also defined a default license, not knowing which one you wanted to use: please update it to whatever license you want to publish your work under  :)

Regards,
Matti