v2.19.0: Auto merge of #755 - kbknapp:issues-740,742,744,753,754, r=kbknapp

v2.19.0 (2016-11-21)

Features

• allows specifying AllowLeadingHyphen style values, but only for specific args vice command wide (c0d70feb, closes #742)

Bug Fixes

• Required Unless: fixes a bug where having required_unless set doesn't work when conflicts are also set (d20331b6, closes #753)
• ZSH Completions: fixes an issue where zsh completions caused panics if there were no subcommands (49e7cdab, closes #754)

Improvements

• Validators: improves the error messages for validators (65eb3385, closes #744)

Documentation

• updates the docs landing page (01e1e33f)
• adds the macro version back to the readme (45eb9bf1)
• fix broken docs links (808e7cee)
• Compatibility Policy: adds an official compatibility policy to (760d66dc, closes #740)
• Contributing: updates the readme to improve the readability and contributing sections (eb51316c)