Consider using a navigation helper to determine current page instead of JavaScript

Currently, the "current page" is identified [using some JavaScript on the client side](https://github.com/cloudfour/drizzle/blob/ea49f40417dd77586bd1000641aa661db48d0283/src/assets/drizzle/scripts/drizzle.js).

It'd be nice if this could be determined during compilation, so while iterating through nav objects we could check for an attribute (`current` or `active` or whatevs) and style things that way.

Low priority, just a thought!