Add estimated reading time

This adds the helper for calculating the reading time of each document and shows it at the top of the page.

Code taken from https://www.perpetual-beta.org/weblog/estimated-reading-time-in-nanoc.html.

---

You need to explicitly enable this in the yaml frontmatter with:

---
reading_time: true
---

TODOS

• We might need to adjust the words per minute count. I tested with 180, but I thought it was too low, so I bumped it to 230.
• Better UX/frontend. Right now it has the same styling as the last updated entry. We may need to update this so that it's shown at the top of the page somewhere.