Transforms should provide list as a first argument

This would allow to parse nested structures more easily. IMHO the best way would be to provide list from most nested value to least nested one, so for file:

```toml
[foo]
bar = { baz = 1 }
```

The transformer argument would look like `[:baz, :bar, :foo]`. This would simplify migration, as current

```elixir
def transform(:baz, value), do: value
```

would look like:

```elixir
def transform([:baz | _], value), do: value
```