Filters for observable modifications

These filters return _functions_ that will perform the specified operation when triggered.

They are effectively shorthand for using `data-event` bindings to modify bound objects. For example,

```html
<a data-event-click='showDetails | toggle'></a>
```

is shorthand for 

```coffeescript 
class MyApp.SomeController extends Batman.Controller
  toggleShowDetails: -> @toggle('showDetails')
```

\+

```html
<a data-event-click='toggleShowDetails'></a>
```

It seems like a nice way to remove boilerplate code. 

If we're feeling really bold, it would be easy to go further and implement filters for the basic Observable operations like 

```html
<a data-event-click='myRecord.property | set "myValue"'></a>
```

or 

```html
<a data-event-click='myRecord.property | unset'></a>
```

(In some cases, `set` is overlapping with a normal `data-bind`, but there are many cases where it allows new things)