ENH: Export PEP8-formatted code

The source generated from export tends to extend beyond 79 characters. {pep8ify, autopep8} automatically format Python code according to PEP8.

When editing an exported pipeline with an editor with syntax checking (e.g. vim and python-mode https://github.com/klen/python-mode#python-libraries ), the PEP8 warnings can be fixed by auto-pep8'ing.
- PEP8 "Style Guide for Python Code"

https://www.python.org/dev/peps/pep-0008/
- https://github.com/spulec/pep8ify
- https://docs.python.org/2/library/2to3.html
- http://python3porting.com/2to3.html
- https://github.com/hhatto/autopep8
- https://github.com/PyCQA/pycodestyle

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- https://github.com/rhiever/tpot/blob/master/tpot/export_utils.py