ability to load variable from file

context.go

@@ -3,6 +3,7 @@ package cli
 import (
 	"errors"
 	"flag"
+	"os"
 	"reflect"
 	"strings"
 	"syscall"
@@ -93,18 +94,26 @@ func (c *Context) IsSet(name string) bool {
 					val = val.Elem()
 				}
 
-				envVarValue := val.FieldByName("EnvVar")
-				if !envVarValue.IsValid() {
-					return
+				filePathValue := val.FieldByName("FilePath")
+				if filePathValue.IsValid() {
+					eachName(filePathValue.String(), func(filePath string) {
+						if _, err := os.Stat(filePath); err == nil {
+							c.setFlags[name] = true
+							return
+						}
+					})
 				}
 
-				eachName(envVarValue.String(), func(envVar string) {
-					envVar = strings.TrimSpace(envVar)
-					if _, ok := syscall.Getenv(envVar); ok {
-						c.setFlags[name] = true
-						return
-					}
-				})
+				envVarValue := val.FieldByName("EnvVar")
+				if envVarValue.IsValid() {
+					eachName(envVarValue.String(), func(envVar string) {
+						envVar = strings.TrimSpace(envVar)
+						if _, ok := syscall.Getenv(envVar); ok {
+							c.setFlags[name] = true
+							return
+						}
+					})
+				}
 			})
 		}
 	}