dump: Unicode support

[vvasuki]

The following two lines:

logging.debug(yml)
logging.debug(yamldown.dump(yml, md))

produces:

DEBUG:2019-04-02 20:53:27,690:csv_to_yaml_md:47 {'लिङ्गम्': 'विश्वास-नामकरणम्', 'विषयः': ['कर्म'], 'पात्राणि': ['विश्वासः', 'जगदीशः', 'रङ्गनायकी', 'वासुकिः'], 'प्रारम्भः': '1982-03-13', 'अन्त्यदिनम्': '1982-03-14', 'रस्यता': '५'}
DEBUG:2019-04-02 20:53:27,691:csv_to_yaml_md:48 ---
"\u0905\u0928\u094D\u0924\u094D\u092F\u0926\u093F\u0928\u092E\u094D": '1982-03-14'
"\u092A\u093E\u0924\u094D\u0930\u093E\u0923\u093F":
- "\u0935\u093F\u0936\u094D\u0935\u093E\u0938\u0903"
- "\u091C\u0917\u0926\u0940\u0936\u0903"
- "\u0930\u0919\u094D\u0917\u0928\u093E\u092F\u0915\u0940"
- "\u0935\u093E\u0938\u0941\u0915\u093F\u0903"
"\u092A\u094D\u0930\u093E\u0930\u092E\u094D\u092D\u0903": '1982-03-13'
"\u0930\u0938\u094D\u092F\u0924\u093E": "\u096B"
"\u0932\u093F\u0919\u094D\u0917\u092E\u094D": "\u0935\u093F\u0936\u094D\u0935\u093E\
  \u0938-\u0928\u093E\u092E\u0915\u0930\u0923\u092E\u094D"
"\u0935\u093F\u0937\u092F\u0903":
- "\u0915\u0930\u094D\u092E"

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Observe how unicode strings are all escaped.