Fixed multiline-import completion

Error occurs at following line.
https://github.com/ujihisa/neco-ghc/blob/826b91e484c5512d3d8c3ff7136c00a5506027f2/autoload/necoghc.vim#L151

additionally,

* replaced printf() by dot-combining from the viewpoint of simplicity and speed

* enabled to do completion in the following case
```haskell
import Foo.Bar ( fun
--             ^^ cannot do completion when preceding space(s) exists.
```