better string interpolation support scoping.

This feature can make config file more clean, the follow is an example config file for angel :

```
root = "$(HOME)/services"
myprogram {
        name = "myprogram"
        root = "$(root)/$(name)"
        exec = "$(root)/$(name)"
        stdout = "$(root)/stdout"
        stderr = "$(root)/stderr"
        delay = 1
}
```