Support for context passed into a partial as an argument

I tried to look this up in the handlebars and mustache documentation, but it seems undocumented. Anyways, partials should be able to accept an argument that establishes a new context:

http://stackoverflow.com/questions/11523331/passing-variables-through-handlebars-partial

Example
======

base.hbs:
```
{{> files files}}
```

files.hbs:
```
{{#each this}}
    <file>{{this}}</file>
{{/each}}
```

data:
```
{
    "files":[
        "file1.txt",
        "file2.txt"
    ]
}
```

Before my pull request, you would get an error that the template "files files" wasn't found. With the pull request, the template renders the property "files" as "this". The result looks like this:

```
<file>file1.txt</file>
<file>file2.txt</file>
```