Add YAML serialization of options

This uses ruamel.yaml. The library seems well-supported, and can do in-place
modification of config files that retains user comments and file structure.

mitmproxy/optmanager.py

@@ -124,10 +124,6 @@ class OptManager(metaclass=_DefaultsMeta):
     def keys(self):
         return set(self._opts.keys())
 
-    @classmethod
-    def default(klass, opt):
-        return copy.deepcopy(klass._defaults[opt])
-
     def reset(self):
         """
             Restore defaults for all options.