Slider: add marks attribute (#15133)

packages/slider/src/marker.js

@@ -0,0 +1,18 @@
+export default {
+  name: 'ElMarker',
+
+  props: {
+    mark: {
+      type: [String, Object]
+    }
+  },
+  render() {
+    let label = typeof this.mark === 'string' ? this.mark : this.mark.label;
+
+    return (
+      <div class="el-slider__marks-text" style={ this.mark.style || {} }>
+        { label }
+      </div>
+    );
+  }
+};