fix(module:radio): add solid button style (#1924)

components/radio/demo/solid.md

@@ -0,0 +1,15 @@
+---
+order: 9
+title:
+  zh-CN: 填底的按钮样式
+  en-US: Solid radio button
+---
+
+## zh-CN
+
+实色填底的单选按钮样式。
+
+## en-US
+
+Solid radio button style.
+

components/radio/demo/solid.ts

@@ -0,0 +1,18 @@
+import { Component } from '@angular/core';
+
+@Component({
+  selector: 'nz-demo-radio-solid',
+  template: `
+    <div>
+      <nz-radio-group [(ngModel)]="radioValue" [nzButtonStyle]="'solid'">
+        <label nz-radio-button nzValue="A">Hangzhou</label>
+        <label nz-radio-button nzValue="B">Shanghai</label>
+        <label nz-radio-button nzValue="C">Beijing</label>
+        <label nz-radio-button nzValue="D">Chengdu</label>
+      </nz-radio-group>
+    </div>
+  `
+})
+export class NzDemoRadioSolidComponent {
+  radioValue = 'A';
+}