Review plugin structure

[AberDerBart]

Right now, the evaluation order of the plugins is not exactly specified. As some sites can be used with multiple plugins, this may cause unpredictable behavior. The plugin system should be changed to have a defined hierarchy for the plugins.

[tstehr]

Also consider removing some plugins that are no longer needed:

• chefkoch: Broken after a website redesign (Chefkoch plugin is broken #5); chefkoch recipes are currently handled by recipe_schema (see Improve handling of keywords in schema plugin #6)
• seriouseats: Can be handled by both recipe_schema and recipe_scrapers; should test which of these produces the better result