feat(module:collapse): support custom header

[SangKa]

Close #382

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #382

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Usage:

import {Component, OnInit} from '@angular/core';

@Component({
  selector: 'nz-demo-collapse-basic',
  template: `
    <nz-collapseset>
      <nz-collapse *ngFor="let panel of panels" [nzActive]="panel.active"
                   [nzDisabled]="panel.disabled">
        <p collapse-title>{{panel.name}}</p>
        <p>{{panel.name}}内容</p>
        <div>papapap</div>
      </nz-collapse>
    </nz-collapseset>
  `,
  styles: []
})
export class NzDemoCollapseBasicComponent implements OnInit {
  panels = [
    {
      active: true,
      name: 'This is panel header 1',
      disabled: false,
      childPanel: [
        {
          active: false,
          name: 'This is panel header 1-1'
        }
      ]
    },
    {
      active: false,
      disabled: true,
      name: 'This is panel header 2'
    },
    {
      active: true,
      disabled: false,
      name: 'This is panel header 3'
    }
  ];

  constructor() {
  }

  ngOnInit() {
  }
}

[coveralls]

Coverage remained the same at 40.752% when pulling 7d5efde on SangKa:close-382 into cf83c96 on NG-ZORRO:master.