Add ability to add paging to tags

Hello!

This adds the ability to add paging to a tag page.  I got a lot of inspiration for the paging code from [metalsmith-collections-paginate](https://github.com/blakeembrey/metalsmith-collections-paginate).

Let me know if this is a welcome change and what else you'd like to see updated so this can get merged upstream.

My paging looks like this for an example: 
![image](https://cloud.githubusercontent.com/assets/322576/4785186/c156f93a-5d75-11e4-926b-3f130fae2f66.png)


Thanks!