Specify output utf-8 encoding explicitly

[Lev135]

I'm working on windows and using unicode syntax in the source code, processed by stylish lead to the following error:

*** Exception: tmp/Main.hs: withFile: invalid argument (invalid character)

This issue can be solved by adding one line into the stylish executable:

IO.withFile path IO.WriteMode $ \h -> do
+ IO.hSetEncoding h IO.utf8
  setNewlineMode h
  IO.hPutStr h new

Nevertheless, maybe this issue can occur somewhere else (if there are another places, where we write a stylish output to the file)? If they are, I think we should add this there too.