Support inheritance in merge operators `|` and `|=`

[LutingWang]

#127 introduces merge operators. However, the current implementation has two problems:

1. Since Dict is a subclass of dict, is it necessary to check by isinstance(other, (Dict, dict))? Wouldn't isinstance(other, dict) be enough?
2. | and |= always return Dict instances, even if the operands may be instances of subclasses. To expatiate, consider the following snippet:

from addict import Dict
class MyDict(Dict):
    def func(self):
        print(self)
a = MyDict()
b = MyDict()
c = a | b
c.func()  # TypeError: 'Dict' object is not callable

Intuitively, we would expect that c is an instance of MyDict but is actually of type Dict. Moreover, since Dict does not have a method called func, when looking up c.func, it returns a new empty Dict, causing the error message to be very misleading.

Will it be possible to return an instance of type self.__class__ in Dict.__or__ and Dict.__ror__?