Specify the utf-8 encoding for the output handle

[Lev135]

The problem is related to #108, however wasn't fixed. I'm working on windows and have a problem with the latest version from haskell/stylish-haskell (closes #428). Maybe printing to the file in utf-8 by default is not what we want by default if system default encoding is not utf-8? However, at least in the case when UnicodeSyntax is enabled it definitely should be so.