Feature request: Continuous indentation

Would it be possible to add a Slim rule to set that certain lines under another line have additional indent, for example that properties belonging to an element are indented additionally?

Example:
```
= form_with_model user do |f|
  / Indented "1 amount", in this case 2 spaces
  = f.text_field :first_name,
      / Indented "2 amounts", in this case 4 spaces
      placeholder: "Name"
```