Packaging update

[crowellel]

Hi all. These changes are in response to issue #518 and bring the package to minimal compliance with packaging standards so we don't get deprecation warnings from pip due to outdated packaging format. Highlights:

• Changed packaging and uploading commands in REAME to allow for building and uploading a source distribution as well as a wheel.
• Added language to setup.py to create universal wheel (necessary for continuing py2.7 compatibility - even if py2.7 is EOL).
• Add minimal pyproject.toml file to meet standards.

This is my first PR, so if I missed something please let me know!

[justinmk]

Thank you for the help here! We definitely need more owners for pynvim, it's suffering from a lack of attention.