Allow config to line up trailing Haddock with field name

Support putting trailing Haddock comments on the same indentation level + 2 spaces, to line up with the record field name.
```hs
{ fooX :: Int
  -- ^ X
, fooY :: Int
  -- ^ Y
}
```

Conversation: https://github.com/fourmolu/fourmolu/pull/124#issuecomment-955293197

Related: #146 