Proposal: plugin system

[yahoo/subscribe-ui-event](https://github.com/yahoo/subscribe-ui-event) provide some function that throttling, caching etc.. by **default**.
We want to the feature by **plugin**.

API Design example:

``` js
const eventObserver = require("ui-event-observer");
const throttling = require("ui-event-observer-plugin-throttling");
eventObserver
.use(throttling({ time: 200 }))
```

The plugin system is similar with [Connect](https://github.com/senchalabs/connect) I think.

Start by reviewing the current eventObserver API and the proposed .use(throttling({ time: 200 })) example. Compare the desired plugin behavior with the Connect plugin model linked in the issue. Done means agreeing on a plugin API and implementing default-extensible behavior for features such as throttling and caching.

Written by the indexing model from the issue text.