Upgrade urfave/cli to v2 (#614)

* Upgrade cli to v2
* Re-vendor

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

@@ -0,0 +1,95 @@
+package cli
+
+import "flag"
+
+// StringFlag is a flag with type string
+type StringFlag struct {
+	Name        string
+	Aliases     []string
+	Usage       string
+	EnvVars     []string
+	FilePath    string
+	Required    bool
+	Hidden      bool
+	TakesFile   bool
+	Value       string
+	DefaultText string
+	Destination *string
+	HasBeenSet  bool
+}
+
+// IsSet returns whether or not the flag has been set through env or file
+func (f *StringFlag) IsSet() bool {
+	return f.HasBeenSet
+}
+
+// String returns a readable representation of this value
+// (for usage defaults)
+func (f *StringFlag) String() string {
+	return FlagStringer(f)
+}
+
+// Names returns the names of the flag
+func (f *StringFlag) Names() []string {
+	return flagNames(f)
+}
+
+// IsRequired returns whether or not the flag is required
+func (f *StringFlag) IsRequired() bool {
+	return f.Required
+}
+
+// TakesValue returns true of the flag takes a value, otherwise false
+func (f *StringFlag) TakesValue() bool {
+	return true
+}
+
+// GetUsage returns the usage string for the flag
+func (f *StringFlag) 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 *StringFlag) GetValue() string {
+	return f.Value
+}
+
+// Apply populates the flag given the flag set and environment
+func (f *StringFlag) Apply(set *flag.FlagSet) error {
+	if val, ok := flagFromEnvOrFile(f.EnvVars, f.FilePath); ok {
+		f.Value = val
+		f.HasBeenSet = true
+	}
+
+	for _, name := range f.Names() {
+		if f.Destination != nil {
+			set.StringVar(f.Destination, name, f.Value, f.Usage)
+			continue
+		}
+		set.String(name, f.Value, f.Usage)
+	}
+
+	return nil
+}
+
+// String looks up the value of a local StringFlag, returns
+// "" if not found
+func (c *Context) String(name string) string {
+	if fs := lookupFlagSet(name, c); fs != nil {
+		return lookupString(name, fs)
+	}
+	return ""
+}
+
+func lookupString(name string, set *flag.FlagSet) string {
+	f := set.Lookup(name)
+	if f != nil {
+		parsed, err := f.Value.String(), error(nil)
+		if err != nil {
+			return ""
+		}
+		return parsed
+	}
+	return ""
+}