v2.5 - Optional 7-day chart background.

* New optional faint sparkline of the last 7 days behind the price
  text. Green when the 7-day move is up, red when down.
* Driven by a new cheeserobot.org/price/price-hist-7d.json endpoint,
  fetched at most once per hour regardless of how often you tap.
* Toggle under Advanced options. On by default.
