feature request: percent change alert

[corpetty]

I'd like to be able to set alerts based on percent price change. This requires some local storage potentially, or it can be naive and only alert within the timeframe of price update initially. Setting this would be a bonus.

[bluppfisk]

This too would require reauthoring the alerts and more stuff in memory. I'll look into it, but again: when I have more time :)

[bluppfisk]

linking to #75