Specify config [sub]destination in Env adapter

[oleiade]

Given a c = Config({'abc': {'123': 'easy as'}) config object, it would be cool to be able to update the c['abc']['123'] value using the env adapter destination selector.

Using a given separator like '.' should make it.

Example:

c = Config({'abc': {'123': 'easy as'}})
env = Env(**{
    'abc.123': 'MY_ENV_VAR'
})

[oleiade]

Fixed by c2b2e06