Pluggable templates.

This is an initial sketch! Hope you have time to take a look.

Actual example with form parsing: https://github.com/cmather/meteor-pluggable-templates-example.

Adds ability to plug functionality into templates. A plugin can have its
own created, rendered and destroyed callbacks, as well as define its own
events.

Example:

```javascript
SimpleForm = function (options) {};
SimpleForm.prototype = {
  constructor: SimpleForm,
  created: function (template) {
    // this points to SimpleForm instance
  },
  rendered: function (template) {},
  destroyed: function (template) {},
  events: {},

  serialize: function () {},
  onSubmit: function () {}
};

SomeOtherPlugin = function (options) {};

Template.myForm.plugins({
  "someName": new SimpleForm({ selector: "form" }),
  "someOtherPlugin": new SomeOtherPlugin({});
});
```