main: switch to urfave/cli/v2

For now this is a drop in replacement for all the flags curently
implemented

Fixes: #171

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

vendor/github.com/urfave/cli/v2/flag_generic.go

@@ -0,0 +1,113 @@
+package cli
+
+import (
+	"flag"
+	"fmt"
+)
+
+// Generic is a generic parseable type identified by a specific flag
+type Generic interface {
+	Set(value string) error
+	String() string
+}
+
+// GenericFlag is a flag with type Generic
+type GenericFlag struct {
+	Name        string
+	Aliases     []string
+	Usage       string
+	EnvVars     []string
+	FilePath    string
+	Required    bool
+	Hidden      bool
+	TakesFile   bool
+	Value       Generic
+	DefaultText string
+	HasBeenSet  bool
+}
+
+// IsSet returns whether or not the flag has been set through env or file
+func (f *GenericFlag) IsSet() bool {
+	return f.HasBeenSet
+}
+
+// String returns a readable representation of this value
+// (for usage defaults)
+func (f *GenericFlag) String() string {
+	return FlagStringer(f)
+}
+
+// Names returns the names of the flag
+func (f *GenericFlag) Names() []string {
+	return flagNames(f.Name, f.Aliases)
+}
+
+// IsRequired returns whether or not the flag is required
+func (f *GenericFlag) IsRequired() bool {
+	return f.Required
+}
+
+// TakesValue returns true of the flag takes a value, otherwise false
+func (f *GenericFlag) TakesValue() bool {
+	return true
+}
+
+// GetUsage returns the usage string for the flag
+func (f *GenericFlag) GetUsage() string {
+	return f.Usage
+}
+
+// GetValue returns the flags value as string representation and an empty
+// string if the flag takes no value at all.
+func (f *GenericFlag) GetValue() string {
+	if f.Value != nil {
+		return f.Value.String()
+	}
+	return ""
+}
+
+// IsVisible returns true if the flag is not hidden, otherwise false
+func (f *GenericFlag) IsVisible() bool {
+	return !f.Hidden
+}
+
+// Apply takes the flagset and calls Set on the generic flag with the value
+// provided by the user for parsing by the flag
+func (f GenericFlag) Apply(set *flag.FlagSet) error {
+	if val, ok := flagFromEnvOrFile(f.EnvVars, f.FilePath); ok {
+		if val != "" {
+			if err := f.Value.Set(val); err != nil {
+				return fmt.Errorf("could not parse %q as value for flag %s: %s", val, f.Name, err)
+			}
+
+			f.HasBeenSet = true
+		}
+	}
+
+	for _, name := range f.Names() {
+		set.Var(f.Value, name, f.Usage)
+	}
+
+	return nil
+}
+
+// Generic looks up the value of a local GenericFlag, returns
+// nil if not found
+func (c *Context) Generic(name string) interface{} {
+	if fs := c.lookupFlagSet(name); fs != nil {
+		return lookupGeneric(name, fs)
+	}
+	return nil
+}
+
+func lookupGeneric(name string, set *flag.FlagSet) interface{} {
+	f := set.Lookup(name)
+	if f != nil {
+		parsed, err := f.Value, error(nil)
+		if err != nil {
+			return nil
+		}
+		return parsed
+	}
+	return nil
+}