intercept line rendering using a 'Painter' (#116)

operation.go

@@ -504,3 +504,13 @@ func (d *DumpListener) OnChange(line []rune, pos int, key rune) (newLine []rune,
 type Listener interface {
 	OnChange(line []rune, pos int, key rune) (newLine []rune, newPos int, ok bool)
 }
+
+type Painter interface {
+	Paint(line []rune, pos int) []rune
+}
+
+type defaultPainter struct{}
+
+func (p *defaultPainter) Paint(line []rune, _ int) []rune {
+	return line
+}

readline.go

@@ -44,6 +44,8 @@ type Config struct {
 	// NOTE: Listener will be triggered by (nil, 0, 0) immediately
 	Listener Listener
 
+	Painter Painter
+
 	// If VimMode is true, readline will in vim.insert mode by default
 	VimMode bool
 
@@ -149,6 +151,10 @@ func (c *Config) SetListener(f func(line []rune, pos int, key rune) (newLine []r
 	c.Listener = FuncListener(f)
 }
 
+func (c *Config) SetPainter(p Painter) {
+	c.Painter = p
+}
+
 func NewEx(cfg *Config) (*Instance, error) {
 	t, err := NewTerminal(cfg)
 	if err != nil {

runebuf.go

@@ -490,11 +490,11 @@ func (r *RuneBuffer) output() []byte {
 		}
 
 	} else {
-		for idx := range r.buf {
-			if r.buf[idx] == '\t' {
+		for _, e := range r.cfg.Painter.Paint(r.buf, r.idx) {
+			if e == '\t' {
 				buf.WriteString(strings.Repeat(" ", TabWidth))
 			} else {
-				buf.WriteRune(r.buf[idx])
+				buf.WriteRune(e)
 			}
 		}
 		if r.isInLineEdge() {