feat: adding new forth color (blue) option

I guess we needed new color, I met a situation where I needed new color option

Let us say
red: failed deployment
yellow: deploying
green: deployed
blue: undeployed (new color)

I bilieve the third color would be able to fit  other scenarios  which must not necessary  be in error,warning,sucess color scheme

badge.go

@@ -34,3 +34,14 @@ func (m *Markdown) GreenBadge(text string) *Markdown {
 func (m *Markdown) GreenBadgef(format string, args ...interface{}) *Markdown {
 	return m.GreenBadge(fmt.Sprintf(format, args...))
 }
+
+// BlueBadge set text with blue badge format.
+func (m *Markdown) blueBadge(text string) *Markdown {
+    m.body = append(m.body, fmt.Sprintf("![Badge](https://img.shields.io/badge/%s-blue)", text))
+    return m
+}
+
+// BlueBadgef set text with blue badge format. It is similar to fmt.Sprintf.
+func (m *Markdown) BlueBadgef(format string, args ...interface{}) *Markdown {
+    return m.blueBadge(fmt.Sprintf(format, args...))
+}

badge_test.go

@@ -47,4 +47,17 @@ func TestMarkdown_RedBadgef(t *testing.T) {
 			t.Errorf("value is mismatch (-want +got):\n%s", diff)
 		}
 	})
+
+	t.Run("success BlueBadgef()", func(t *testing.T) {
+		t.Parallel()
+
+		m := NewMarkdown(io.Discard)
+		m.BlueBadgef("%s", "Hello")
+		want := []string{"![Badge](https://img.shields.io/badge/Hello-blue)"}
+		got := m.body
+
+		if diff := cmp.Diff(want, got); diff != "" {
+			t.Errorf("value is mismatch (-want +got):\n%s", diff)
+		}
+	})
 }