Move link and indicator rendering to configurable class

Hey again,

While reskinning our older app, we wanted to avoid changing things where possible. We've been using your gem for a while, but needed to adjust the rendering to make it suit our new layouts.

Rather than tweaking it for our own purposes, I thought I'd share this change!

The goal of this change is to extract the link generation to a class that can be chosen by the user. By default it uses a class that mimics the existing behaviour.

Thanks
p.s. no tests, ugh!