Support named arguments with spaces and special characters

Hi, davidchambers,
Can named arguments, object keys with spaces ('avg val') or special characters (#, _ etc) be supported?

``` javascript
var user = {name: "Dolly", age: 24, 'in co': 23, '#':1}
undefined
"{name} - {age}".format(user)
"Dolly - 24"
"{name} - {age} - {in co}".format(user)
"Dolly - 24 - {in co}"
"{name} - {age} - {#}".format(user)
"Dolly - 24 - {#}"
```
