minor bugfix: comparing plugins for caching purposes now done against X-Samizdat-Method response header

Configurable strategy

Implementing a strategy configurable via an external `config.js` file.

See merge request !28

WIP #70: config.js can now configure which plugins are loaded, and what order they are going to be used in

Service Worker and plugin config in a separate config file

See merge request !27

Documentation

See merge request !26

gateway-ipns bugfix: gateways are now correctly randomized from the pre-configured list, without affecting the list itself