Feature: prefix and suffix

[douglasmiranda]

Maybe it would be useful to implement a prefix and suffix optional feature, like:

Envs:

DJANGO_DEBUG=False
DJANGO_SECRET=abc

>>> from gconfigs import GConfigs
>>> from gconfigs.backends import LocalEnv

>>> envs = GConfigs(backend=LocalEnv, prefix="DJANGO_")
>>> envs.as_bool("DEBUG")
False
>>> envs("SECRET")
"abc"

With context managers:

>>> from gconfigs import GConfigs
>>> from gconfigs.backends import LocalEnv

>>> envs = GConfigs(backend=LocalEnv)
>>> with envs.prefix("DJANGO"):
...    envs("SECRET")
...
"abc"

Well, suffix would do the same, but you know... following the suffix logic.