Update to urfave/cli introduces breaking changes

[eking-isp]

This also broke swaggo/swag#556, but you can no longer build this tool with the latest CLI since it recently introduced a breaking change:

..\mikunalpha\goas\main.go:13:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
..\mikunalpha\goas\main.go:18:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
..\mikunalpha\goas\main.go:23:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
..\mikunalpha\goas\main.go:28:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
..\mikunalpha\goas\main.go:33:14: cannot use cli.BoolFlag literal (type cli.BoolFlag) as type cli.Flag in array or slice literal:
        cli.BoolFlag does not implement cli.Flag (Apply method has pointer receiver)
..\mikunalpha\goas\main.go:40:23: c.GlobalString undefined (type *cli.Context has no field or method GlobalString)
..\mikunalpha\goas\main.go:40:120: c.GlobalBool undefined (type *cli.Context has no field or method GlobalBool)
..\mikunalpha\goas\main.go:45:26: c.GlobalString undefined (type *cli.Context has no field or method GlobalString)

[mikunalpha]

@eking-isp
Do you recommend using urfave/cli v2 ?

[eking-isp]

I do, yeah, at least until an upgrade plan is in place for it