## Add dynamic instrumentation to emitters
  
  `shimmer` does a bunch of the work necessary to wrap other methods in
  a wrapper you provide:
  
  ```javascript
  var EventEmitter = require('events').EventEmitter;
  var wrapEmitter = require('emitter-listener');
  
  var ee = new EventEmitter();
  
  var id = 0;
  
  wrapEmitter(
    ee,
    function mark(listener) {
      listener.id = id++;
    },
    function prepare(listener) {
      console.log('listener id is %d', listener.id);
    }
  );
  ```
  
  ### Mandatory disclaimer
  
  There are times when it's necessary to monkeypatch default behavior in
  JavaScript and Node. However, changing the behavior of the runtime on the fly
  is rarely a good idea, and you should be using this module because you need to,
  not because it seems like fun.
  
  #### wrapEmitter(emitter, mark, prepare)
  
  Wrap an EventEmitter's event listeners. Each listener will be passed to
  `mark` when it is registered with `.addListener()` or `.on()`, and then
  each listener is passed to `prepare` to be wrapped before it's called
  by the `.emit()` call. `wrapListener` deals with the single listener
  vs array of listeners logic, and also ensures that edge cases like
  `.removeListener()` being called from within an `.emit()` for the same
  event type is handled properly.
  
  The wrapped EE can be restored to its pristine state by using
  emitter.__unwrap(), but this should only be used if you *really* know
  what you're doing.