include CSS file as base styling

I've been using d3-tip in a few different projects and I keep copy/pasting the same css into different places. It would be convenient to have a base styling by default (like most other javascript packages) and make it easy for people to overwrite or completely change however they'd like. Eventually, we could throw the css + javascript on cdnjs so that users only have to add something like this to the html:

```html
<script src="//cdnjs.cloudflare.com/ajax/libs/d3-tip/X.Y.Z/d3-tip.min.js"></script>
<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/d3-tip/X.Y.Z/d3-tip.min.css"></script>
```

Of course, users don't *have* to include the CSS for this module to work, but it provides a good starting point when you're just getting started.