[Feature Request] Add new check to validate undefined variable

[moylop260]

Steps to reproduce

1. Running the following code

if False:
    new_variable = "hello"
print new_variable

We have the following error:

Traceback (most recent call last):
  File "example.py", line 3, in <module>
    print new_variable
NameError: name 'new_variable' is not defined

Current behavior

If we run a pylint to check if this error is detected early we don't have a check for this case.

Expected behavior

Emit a check similar to W0631 undefined-loop-variable - Using possibly undefined loop variable 'new_variable' but for the if section.

pylint --version output

Latest master sha for astroid and pylint

[degustaf]

Duplicate of #85.