Support attribute list 

Given:

```terraform
#main.tf
locals {
  a = "foo"
  b = "bar"
  c = {"nums":[1,2]}
}
```

It would be nice to be able to do something like:

```bash
hcledit -f main.tf attribute list locals
```

to get:

```
locals.a
locals.b
locals.c.nums[0]
locals.c.nums[1]
```