Break out URLs

[MichaelCurrin]

Sample:

[![NPM Version][npm-image]][npm-url]
[![Build Status][travis-image]][travis-url]
[![Downloads Stats][npm-downloads]][npm-url]

<!-- Markdown link & img dfn's -->
[npm-image]: https://img.shields.io/npm/v/datadog-metrics.svg?style=flat-square
[npm-url]: https://npmjs.org/package/datadog-metrics
[npm-downloads]: https://img.shields.io/npm/dm/datadog-metrics.svg?style=flat-square
[travis-image]: https://img.shields.io/travis/dbader/node-datadog-metrics/master.svg?style=flat-square
[travis-url]: https://travis-ci.org/dbader/node-datadog-metrics

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URLs at the bottom of the page in the original but it is more convenient to keep them just under badges