Add support for checking for new coupons at user defined intervals

[douglasparker]

Default to once a day.

[douglasparker]

This may not be worth implementing because safeway.com uses a Web Application Firewall (WAF) that will trigger a captcha and potentially block what it deems as bot-like behavior. This increases the risk of being flagged when automated to run more frequently than necessary.

Furthermore, by requiring people to visit the page manually, they're less likely to assume everything worked fine when it didn't; resulting in spending more than intended.

I could implement this to run infrequently and disabled by default- with a disclaimer when enabling. However, automatic updates will result in the behavior being disabled again.

Regardless, everyone should be manually checking that their coupons are clipped before going shopping.