Feat: Add epilog after help text

Similar to the Description at the top of the
help text an Epilog is added at the bottom.

Resolves #189

README.md

@@ -462,6 +462,9 @@ Options:
 
 ### Description strings
 
+A descriptive message can be added at the top of the help text by implementing
+a `Description` function that returns a string.
+
 ```go
 type args struct {
 	Foo string
@@ -487,6 +490,35 @@ Options:
   --help, -h             display this help and exit
 ```
 
+Similarly an epilogue can be added at the end of the help text by implementing
+the `Epilogue` function.
+
+```go
+type args struct {
+	Foo string
+}
+
+func (args) Epilogue() string {
+	return "For more information visit github.com/alexflint/go-arg"
+}
+
+func main() {
+	var args args
+	arg.MustParse(&args)
+}
+```
+
+```shell
+$ ./example -h
+Usage: example [--foo FOO]
+
+Options:
+  --foo FOO
+  --help, -h             display this help and exit
+
+For more information visit github.com/alexflint/go-arg
+```
+
 ### Subcommands
 
 *Introduced in version 1.1.0*

parse.go

@@ -134,6 +134,7 @@ type Parser struct {
 	config      Config
 	version     string
 	description string
+	epilogue    string
 
 	// the following field changes during processing of command line arguments
 	lastCmd *command
@@ -155,6 +156,14 @@ type Described interface {
 	Description() string
 }
 
+// Epilogued is the interface that the destination struct should implement to
+// add an epilogue string at the bottom of the help message.
+type Epilogued interface {
+	// Epilogue returns the string that will be printed on a line by itself
+	// at the end of the help message.
+	Epilogue() string
+}
+
 // walkFields calls a function for each field of a struct, recursively expanding struct fields.
 func walkFields(t reflect.Type, visit func(field reflect.StructField, owner reflect.Type) bool) {
 	walkFieldsImpl(t, visit, nil)
@@ -236,6 +245,9 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 		if dest, ok := dest.(Described); ok {
 			p.description = dest.Description()
 		}
+		if dest, ok := dest.(Epilogued); ok {
+			p.epilogue = dest.Epilogue()
+		}
 	}
 
 	return &p, nil

usage.go

@@ -290,6 +290,10 @@ func (p *Parser) writeHelpForSubcommand(w io.Writer, cmd *command) {
 			printTwoCols(w, subcmd.name, subcmd.help, "", "")
 		}
 	}
+
+	if p.epilogue != "" {
+		fmt.Fprintln(w, "\n"+p.epilogue)
+	}
 }
 
 func (p *Parser) printOption(w io.Writer, spec *spec) {

usage_test.go

@@ -285,6 +285,37 @@ Options:
 	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
 }
 
+type epilogued struct{}
+
+// Epilogued returns the epilogue for this program
+func (epilogued) Epilogue() string {
+	return "For more information visit github.com/alexflint/go-arg"
+}
+
+func TestUsageWithEpilogue(t *testing.T) {
+	expectedUsage := "Usage: example"
+
+	expectedHelp := `
+Usage: example
+
+Options:
+  --help, -h             display this help and exit
+
+For more information visit github.com/alexflint/go-arg
+`
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &epilogued{})
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp[1:], help.String())
+
+	var usage bytes.Buffer
+	p.WriteUsage(&usage)
+	assert.Equal(t, expectedUsage, strings.TrimSpace(usage.String()))
+}
+
 func TestUsageForRequiredPositionals(t *testing.T) {
 	expectedUsage := "Usage: example REQUIRED1 REQUIRED2\n"
 	var args struct {