Enable helmfile-diff to pass the output format to helm-diff (#1784)

* tests: fix vagrant test run

* feat: added an option to specify the different diff output format

* renamed diff-output to output

* renamed diff-output to output

Co-authored-by: Andrey Tuzhilin <andrey@zelf.co>

main.go

@@ -225,6 +225,11 @@ func main() {
 					Value: 0,
 					Usage: "output NUM lines of context around changes",
 				},
+				cli.StringFlag{
+					Name:  "output",
+					Value: "",
+					Usage: "output format for diff plugin",
+				},
 			},
 			Action: action(func(a *app.App, c configImpl) error {
 				return a.Diff(c)
@@ -441,6 +446,11 @@ func main() {
 					Value: 0,
 					Usage: "output NUM lines of context around changes",
 				},
+				cli.StringFlag{
+					Name:  "output",
+					Value: "",
+					Usage: "output format for diff plugin",
+				},
 				cli.BoolFlag{
 					Name:  "detailed-exitcode",
 					Usage: "return a non-zero exit code 2 instead of 0 when there were changes detected AND the changes are synced successfully",
@@ -855,6 +865,10 @@ func (c configImpl) Context() int {
 	return c.c.Int("context")
 }
 
+func (c configImpl) DiffOutput() string {
+	return c.c.String("output")
+}
+
 func (c configImpl) SkipCleanup() bool {
 	return c.c.Bool("skip-cleanup")
 }