[feature request] Support for defined number of blank lines between top-level declarations

After using [elm-format](https://github.com/avh4/elm-format) (the standard elm formatter) I realized that it would be convenient to have the option to enable a defined number of blank lines between top-level declarations, to avoid having places where there is inconsistent spacing between declarations and minimize diffs when refactoring and adding new code.

(I looked through issues and the default config file for an already existing option to enable this, without finding anything)

Hopefully with this option, stylish-haskell would change this:
```haskell
fun :: Num a => a -> a -> a
fun a b = a + b
gun :: t
gun = gun

i :: Integer
i = 0

o :: Integer
o = 0
```

To this (assuming a 2 blank-line separation between declarations):
```haskell
fun :: Num a => a -> a -> a
fun a b = a + b

gun :: t
gun = gun

i :: Integer
i = 0

o :: Integer
o = 0
```

Start by locating stylish-haskell's configuration handling and the formatter code that separates top-level declarations. Read the existing formatting tests before deciding where the blank-line option belongs. Done means the option produces the two example layouts consistently and has coverage for the configured spacing.

Written by the indexing model from the issue text.