Cleaned up protocol in src/sources/protocol.lisp

* src/sources/conditions.lisp:
* src/sources/protocol.lisp (notify): removed; sinks are now defined
  in the options package
  (initialize): new generic function; initialize a configuration
  option source
  (process): new generic function; process content of a configuration
  option source and provide resulting options to a sink
  (define-condition-translating-method initialize): default behavior;
  translate conditions to `initialization-error'
  (initialize t t): default behavior; do nothing by default
  (define-condition-translating-method process): default behavior;
  translate conditions to `processing-error'
* src/sources/package.lisp (package options.sources): added exported
  symbols source-condition, source-condition-source, sink-condition,
  sink-condition-sink, initialization-error, processing-error,
  initialize and process