feat: make small improvements to optionparser

1. Add a Coda field to OptionParser.  The user can put text here that
   will appear at the bottom of a help message.  (This is especially
   nice for links pointing to further information or where to file bug
   reports.)
2. Make a few small changes pointed out by staticcheck.  (Do not use
   uppercase letters at the start of an error, and avoid a loop when
   using `append`.)
3. Change the help message to go better with my preferences.

internal/optionparser/optionparser.go

@@ -18,11 +18,12 @@ type command struct {
 }
 
 // OptionParser contains the methods to parse options and the settings to
-// influence the output of --help. Set the Banner for usage info, set Start and
-// Stop for output of the long description text.
+// influence the output of --help. Set the Banner and Coda for usage info, set
+// Start and Stop for output of the long description text.
 type OptionParser struct {
 	Extra    []string
 	Banner   string
+	Coda     string
 	Start    int
 	Stop     int
 	options  []*allowedOptions
@@ -203,9 +204,7 @@ func set(obj *allowedOptions, hasNoPrefix bool, param string) {
 	}
 	if obj.stringslice != nil {
 		eachParam := strings.Split(param, ",")
-		for _, v := range eachParam {
-			*obj.stringslice = append(*obj.stringslice, v)
-		}
+		*obj.stringslice = append(*obj.stringslice, eachParam...)
 	}
 	if obj.functionNoArgs != nil {
 		obj.functionNoArgs()
@@ -327,7 +326,7 @@ func (op *OptionParser) ParseFrom(args []string) error {
 			}
 
 			if option == nil {
-				return fmt.Errorf("Unknown option %s", ret.argument)
+				return fmt.Errorf("unknown option %s", ret.argument)
 			}
 
 			// the parameter in ret.param is only set by `splitOn()` when used with
@@ -359,7 +358,7 @@ func (op *OptionParser) ParseFrom(args []string) error {
 					// parameter expected
 					if !option.optional {
 						// No parameter found but expected
-						return fmt.Errorf("Parameter expected but none given %s", ret.argument)
+						return fmt.Errorf("parameter expected but none given %s", ret.argument)
 					}
 				}
 				set(option, ret.negate, "")
@@ -428,6 +427,9 @@ func (op *OptionParser) Help() {
 			formatAndOutput(op.Start, op.Stop, "", "", "", "", cmd.name, lines)
 		}
 	}
+	if op.Coda != "" {
+		fmt.Println(op.Coda)
+	}
 }
 
 // NewOptionParser initializes the OptionParser struct with sane settings for Banner,
@@ -440,6 +442,6 @@ func NewOptionParser() *OptionParser {
 	a.Stop = 79
 	a.short = map[string]*allowedOptions{}
 	a.long = map[string]*allowedOptions{}
-	a.On("-h", "--help", "Show this help", func() { a.Help(); os.Exit(0) })
+	a.On("-h", "--help", "Print this help and exit", func() { a.Help(); os.Exit(0) })
 	return a
 }