Parse files

I liked the format you used in the README a lot better, so I updated the plugin to allow it. The following are now all valid

```
<gist>3431249</gist>
<gist>3431249.js</gist>
<gist>3431249?file=style.css</gist>
<gist>3431249.js?file=style.css</gist>
```

I don't know why you'd ever really want to include the .js in the gist index, but as my original PR used it, I thought it best not to break that functionality.

If you merge this, you can close #6