Support for multibyte for "chupa-text" command

Because the following error occurred:

% cat multibyte.txt
札幌
% chupa-text multibyte.txt
/.../lib/ruby/2.1.0/json/common.rb:285:in `encode': "\xE6" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
        from /.../lib/ruby/2.1.0/json/common.rb:285:in `generate'
        from /.../lib/ruby/2.1.0/json/common.rb:285:in `pretty_generate'
        from /.../lib/ruby/gems/2.1.0/gems/chupa-text-1.0.4/lib/chupa-text/formatters/json.rb:40:in `format_finish'
        from /.../lib/ruby/gems/2.1.0/gems/chupa-text-1.0.4/lib/chupa-text/command/chupa-text.rb:46:in `run'
        from /.../lib/ruby/gems/2.1.0/gems/chupa-text-1.0.4/lib/chupa-text/command/chupa-text.rb:25:in `run'
        from /.../lib/ruby/gems/2.1.0/gems/chupa-text-1.0.4/bin/chupa-text:21:in `<top (required)>'
        from /.../bin/chupa-text:23:in `load'
        from /.../bin/chupa-text:23:in `<main>'