Release 1.21.0

🚧 Watch out! There's a known regression in this release! #850 🚧

Fixed

• Fix using "slice" flag types with EnvVar in urfave/cli/pull/687 via @joshuarubin
• Fix regression of SkipFlagParsing behavior in urfave/cli/pull/697 via @jszwedko
• Fix handling ShortOptions and SkipArgReorder in urfave/cli/pull/686 via @baude
• Fix args reordering when bool flags are present in urfave/cli/pull/712 via @windler
• Fix parsing of short options in urfave/cli/pull/758 via @vrothberg

Added / Changed

• Added "required flags" support in urfave/cli/pull/819 via @lynncyrin
• Cleaned up help output in urfave/cli/pull/664 via @maguro
• Case is now considered when sorting strings in urfave/cli/pull/676 via @rliebz
• Backport JSON InputSource to v1 in urfave/cli/pull/598 via @jszwedko
• Allow more customization of flag help strings in urfave/cli/pull/661 via @rliebz
• Allow custom ExitError handler function in urfave/cli/pull/628 via @phinnaeus
• Allow loading a variable from a file in urfave/cli/pull/675 via @jmccann
• Allow combining short bool names in urfave/cli/pull/684 via @baude