Specify output utf-8 encoding explicitly

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:
```diff
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.