Enhance shm/goto-where

It seemed like good idea to me to be able to configure shm/goto-where to insert a little bit more indentation, especially considering that shm/tab does not indent point like this (`_|_` being the point):

given
```haskell
f x = ...
  where
  _|_
```
becomes after `shm/tab`
```haskell
f x = ...
  where
    _|_
```

If you don't agree with this change at least the test for shm/goto-where may be useful.