Add an example for an e-commerce site!

[perrygoy]

SauceLabs maintains an e-commerce test site at https://www.saucedemo.com/, which would allow for much less contrived example tests.

H/t to @mdmintz for the suggestion!

[perrygoy]

This might be a good place to have some sort of mock example, somehow, as mentioned in ScreenPyHQ/screenpy#35.