Attribute and method store/compute meta features of StackingRegressor

[rasbt]

This implementation is analogous to how the respective methods were implemented for the StackingCVRegressor in #294

[pep8speaks]

Hello @rasbt! Thanks for submitting the PR.

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

Line 78:80: E501 line too long (80 > 79 characters)

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

Line 60:80: E501 line too long (80 > 79 characters)

[coveralls]

Coverage increased (+0.04%) to 90.936% when pulling c525f36 on store_meta into fa5d1f7 on master.