WIP: display_format for style
Unfinished at the moment. Just wanted to push something up since there are other PRs that could build on this.

This moves the logic of "what value should I display for this cell?" from the Template to the `Styler` class itself. The two biggest benefits are
1. User extensibility: Users provide formatting functions (via an API that's TBD) which are called upon the data values when we `translate`.
2. Maintainability: Instead of complex logic in the template, we have (slightly) less complex logic in the class itself.
