Event emitter component.

$ component install component/emitter

The `Emitter` may also be used as a mixin. For example
a "plain" object may become an emitter, or you may
extend an existing prototype.

As an `Emitter` instance:

js
var Emitter = require('emitter');
var emitter = new Emitter;
emitter.emit('something');

As a mixin:

js
var Emitter = require('emitter');
var user = { name: 'tobi' };
Emitter(user);

user.emit('im a user');

As a prototype mixin:

js
var Emitter = require('emitter');
Emitter(User.prototype);

Register an `event` handler `fn`.

Register a single-shot `event` handler `fn`,
removed immediately after it is invoked the
first time.

Emit an `event` with variable option args.

Return an array of callbacks, or an empty array.

Check if this emitter has `event` handlers.