Add MarginWriter to avoid code repetition

margin.go

@@ -0,0 +1,53 @@
+package gold
+
+import (
+	"io"
+)
+
+type MarginWriter struct {
+	w  io.Writer
+	pw *PaddingWriter
+	iw *IndentWriter
+}
+
+func NewMarginWriter(ctx RenderContext, w io.Writer, rules ElementStyle) *MarginWriter {
+	bs := ctx.blockStack
+
+	var indent uint
+	var margin uint
+	if rules.Indent != nil {
+		indent = *rules.Indent
+	}
+	if rules.Margin != nil {
+		margin = *rules.Margin
+	}
+
+	pw := &PaddingWriter{
+		Padding: bs.Width(ctx),
+		PadFunc: func(wr io.Writer) {
+			renderText(w, rules, " ")
+		},
+		Forward: &AnsiWriter{
+			Forward: w,
+		},
+	}
+	iw := &IndentWriter{
+		Indent: indent + margin,
+		IndentFunc: func(wr io.Writer) {
+			renderText(w, bs.Parent().Style, " ")
+		},
+		Forward: &AnsiWriter{
+			Forward: pw,
+		},
+	}
+
+	return &MarginWriter{
+		w:  w,
+		pw: pw,
+		iw: iw,
+	}
+}
+
+func (w *MarginWriter) Write(b []byte) (int, error) {
+	return w.iw.Write(b)
+}