Make "=>" function binding optional

Not sure if theres a particular reason for wanting all the tag `options.content` functions to use function binding with the fat arrow. But its not too difficult to change it without any breaking changes.

```coffee
@div id: 'a', =>
  @div id: 'b'
# becomes
@div id: 'a', ->
  @div id: 'b'
```

---

and in javascript, this:
```js
return this.div({
  id: 'a'
}, (function(_this) {
  return function() {
    return _this.div({
      id: 'b'
    });
  };
})(this));
```

can become this:
```js
return this.div({
  id: 'a'
}, function() {
  return this.div({
    id: 'b'
  });
});
```

the only benefit I can think of for `=>`, is that `_this` gets minified into a single character, and in a really large view, that can save bytes.