Add support for fees

[kennethormandy]

• [x] Create fee table on install
• [x] Remove fee table on uninstall
• [x] Add fee type and value
• [x] Actually use fee in checkout flow
• [x] Limit to one fee for Lite (could also consider limiting to flat fee only, and keep percent fee as an upgrade—might allow you to add more than one, but show the extras as disabled on the settings page? Not going as far as letting the upgrade users enable / disable fees, but in the checkout flow we would only iterate over all of them for the Pro plugin)