Setup a mlxtend.image placeholder module

[rasbt]

Description

Sets up a new mlxtend.image module as a placeholder for functions to be implemented (cc @vmirly)

Related issues or pull requests

None

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• Ran nosetests ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., nosetests ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend

[pep8speaks]

Hello @rasbt! Thanks for submitting the PR.

• There are no PEP8 issues in the file mlxtend/image/__init__.py !

• There are no PEP8 issues in the file mlxtend/image/extract_face_landmarks.py !

• There are no PEP8 issues in the file mlxtend/image/tests/test_extract_face_landmarks.py !

• There are no PEP8 issues in the file setup.py !