Configurable plugin set and strategy

Since we already have a configuration system for the Service Worker (#69 (closed)), we can implement a way to configure:

• what plugins are loaded
• what is the content retrieval strategy (that is, what order are the plugins being deployed to fetch the content)
  • simple order (expressed as an array)
  • support for more complex structures with steps involving "any of these plugins" approaches (expressed as an array inside the simple ordered array, perhaps?)