add `.assign()`

Add a .assign(path, val, missing = None) convenience method.
Can now use the missing callable to backfill missing data structures.

>>> from glom_dict import GlomDict
>>> my_dict = GlomDict()
>>> my_dict.assign('a.b.c', 'hi', missing=dict)
{'a': {'b': {'c': 'hi'}}}

Python 3.6 support

Add support for python 3.6.2 and above