Properly split and parse requirements lines

[techalchemy]

• Split out markers and local paths
• Fixes extras_require and length of name > 5 fails. #870 and How to install setup.py optional dependencies for a package with extras? #1083 and In pipfile editable install dependencies was strange. #858 and pienv can't detect environment markers from requirements.txt #848
• Also splits markers properly using pip's approach
• The requirements parser we use really needs a lot of help

[techalchemy]

This may need the fix I just pushed to #1091 in order for appveyor to pass

[kennethreitz]

✨🍰✨