[Feature request]: White space format support.

```haskell
main = putStrLn "Hello World"
```

This is a valid statement in haskell. But there is no change when I type `stylish-haskell -i helloworld.hs`.

I was wondering if it is possible to add white space formatting support in stylish-haskell.

Start by reproducing the reported command, `stylish-haskell -i helloworld.hs`, with the whitespace-heavy Haskell statement shown in the issue. Read the formatter entry points to determine where whitespace formatting would belong. Done means the command applies a defined whitespace formatting behavior to valid Haskell input.

Written by the indexing model from the issue text.