Feature: (issue_1451) customized slice flag separator (#1546)

* feat: customized slice flag separator

* feat: modify go doc

* feat: update unit test

testdata/godoc-v2.x.txt

@@ -316,6 +316,8 @@ type App struct {
 	// cli.go uses text/template to render templates. You can
 	// render custom help text by setting this variable.
 	CustomAppHelpTemplate string
+	// SliceFlagSeparator is used to customize the separator for SliceFlag, the default is ","
+	SliceFlagSeparator string
 	// Boolean to enable short-option handling so user can combine several
 	// single-character bool arguments into one
 	// i.e. foobar -o -v -> foobar -ov