Track and report on multiple items

Add a config that contains multiple items to track, and the
price-scraper will iterate through each and track to that new price.
The controller reports back if the price has been updated (changed).
Establish tracked-item mongoose model

Instead of using a config file to store the tracked items, switch to
use a mongo/mongoose model.  Pull the logic which scrapes the sites
from the controller to a library file.  Build a controller to expose
tracked items, and for now, expose an update API to update the prices.