Indent correctly for more cases

Prevent indention for lines starting with a label.
Indent remaining lines by four spaces if they are indented at any level.

This change handles this code:
```
;;Beginning
 mov r7, #5
        lsl r7, #3
   some_label: .ascii "hello world"
```
...which is prettifyed as:
```
;;Beginning
    mov r7, #5
    lsl r7, #3
some_label: .ascii "hello world"
```