Add an issue template

[itaisteinherz]

We should add a template for issue to save time triaging new ones. We could add a template for PRs as well, but I'm not sure how useful that will be.

This is what I currently have in mind:

## Issue description

...

## Usage information

- [ ] Publishing to a private registry.
- [ ] Private package.
- [ ] Scoped package.

## System information

np - x.x.x
Node.js - x.x.x
npm - x.x.x
Git - x.x.x

Open to suggestions on how this can be improved to be more informative and helpful 🙃

[sindresorhus]

👍 can you do a PR with this?

[itaisteinherz]

👍 can you do a PR with this?

Sure 👌🏻

About the PR template, I think that currently one of the main problems I see is that many people create a PR which modifies np's current flow to match their own, instead of creating an issue first, so that we can help them resolve their issue instead of let them down by telling them that their PR isn't going to get merged. This is what I have in mind:

**Important: In case your PR adds new non-trivial functionality or modifies the current behavior of `np`, please create an issue first to discuss it so that it won't get dismissed and closed.**