Automatically add `-- ` when pressing enter in a comment

A minor but nice feature I would like to propose is to automatically add `-- ` on the new line when pressing "enter" in a comment/documentation.

Example (cursor position represented by `[]`) below.

# Current behavior
```haskell
-- | Somme doc[]
foo = "hello world"
```
Press enter
```haskell
-- | Somme doc
[]
foo = "hello world"
```

# Proposed behavior
```haskell
-- | Somme doc[]
foo = "hello world"
```
Press enter
```haskell
-- | Somme doc
-- []
foo = "hello world"
```

I think this default behavior makes much more sense for developers.

To be discussed: should we do the same when in a regular comment? I'd say "no" but I am not strongly opinionated on this one.

Cheers