Option to pass orig features to meta regressor in stackingregressor

[rasbt]

Description

Adding support for merging the meta features with the original input features in StackingRegressor (via use_features_in_secondary) like it is already supported in the other Stacking classes.

Related issues or pull requests

Fixes #418

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.

• In the file mlxtend/regressor/stacking_regression.py, following are the PEP8 issues :

Line 69:80: E501 line too long (80 > 79 characters)

[coveralls]

Coverage increased (+0.06%) to 91.24% when pulling 877c021 on stackingclassifier-metafeatures into c027a1c on master.