Make aligning of import lists configurable

Currently, `stylish-haskell` aligns import lists so that they all start in the same column. When adding new import statements with longer module names this thus changes all import statements with import lists (and bloats file diffs). Can we have this configurable?

**Example**

Starting with:

``` haskell
import Foo (foo)
```

Adding one import:

``` haskell
import Foo (foo)
import Baaaaaaar
```

Running `stylish-haskell`:

``` haskell
import Baaaaaaar
import Foo (foo)
```

What I'd like to see:

``` haskell
import Baaaaaaar
import Foo (foo)
```