Correct sections context

Hey,

Handlebars should support sections in a slightly different way than is implemented. The comments say it's an 'implicit if', however that's not just the case, it also provides the given key as the new context.

Given this data
```
{
  name: "User",
  order: {
    number: 1234
  }
}
```

```
{{#if name}}
{{name}}
{{/if}}
```
Should be the same as
```
{{#name}}
{{.}}
{{/name}}
```

The same thing happens with an object. If you're calling a value in the section, the context should be that of the given key.

```
{{#order}}
Number: {{number}}
{{/order}}
```
Note that's not `{{order.number}}`, since the context is actually `order`.