Unicode chars are not supported

[Aliaksei-Tatarynchyk]

If file which is read by shaper contains utf8 char, e.g. русский текст, it fails with following exception:

resolver.py:145: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  if value == u'':
Traceback (most recent call last):
  File "d:\env\python\python27\lib\runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "d:\env\python\python27\lib\runpy.py", line 72, in _run_code
    exec code in run_globals
  File "d:\Env\Python\Python27\Scripts\shaper.exe\__main__.py", line 9, in <module>
  File "d:\env\python\python27\lib\site-packages\shaper\cli.py", line 144, in main
    libs.parser.write(tree, arguments.out)
  File "d:\env\python\python27\lib\site-packages\shaper\libs\parser.py", line 102, in write
    parser_class().write(data, path)
  File "d:\env\python\python27\lib\site-packages\shaper\libs\parser.py", line 186, in write
    allow_unicode=True,
  File "d:\env\python\python27\lib\site-packages\yaml\__init__.py", line 202, in dump
    return dump_all([data], stream, Dumper=Dumper, **kwds)
  File "d:\env\python\python27\lib\site-packages\yaml\__init__.py", line 190, in dump_all
    dumper.represent(data)
  File "d:\env\python\python27\lib\site-packages\yaml\representer.py", line 29, in represent
    self.serialize(node)
  File "d:\env\python\python27\lib\site-packages\yaml\serializer.py", line 54, in serialize
    self.serialize_node(node, None, None)
  File "d:\env\python\python27\lib\site-packages\yaml\serializer.py", line 108, in serialize_node
    self.serialize_node(value, node, key)
  File "d:\env\python\python27\lib\site-packages\yaml\serializer.py", line 90, in serialize_node
    style=node.style))
  File "d:\env\python\python27\lib\site-packages\yaml\emitter.py", line 115, in emit
    self.state()
  File "d:\env\python\python27\lib\site-packages\yaml\emitter.py", line 413, in expect_block_mapping_simple_value
    self.expect_node(mapping=True)
  File "d:\env\python\python27\lib\site-packages\yaml\emitter.py", line 243, in expect_node
    self.process_tag()
  File "d:\env\python\python27\lib\site-packages\yaml\emitter.py", line 474, in process_tag
    self.style = self.choose_scalar_style()
  File "d:\env\python\python27\lib\site-packages\yaml\emitter.py", line 497, in choose_scalar_style
    self.analysis = self.analyze_scalar(self.event.value)
  File "d:\env\python\python27\lib\site-packages\yaml\emitter.py", line 653, in analyze_scalar
    if scalar.startswith(u'---') or scalar.startswith(u'...'):
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 434: ordinal not in range(128)