feat(module:modal): provide open & close & setConfirmLoading function

[giscafer]

use #modal.open() instead of set nzVisible=true
use #modal.close() instead of set nzVisible=false
use #modal.setConfirmLoading(loading:boolean) instead of set confirmLoading

close #118

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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Coverage remained the same at 52.233% when pulling 534b329 on giscafer:modal-open-close-loading into 81cbae9 on NG-ZORRO:master.

[vthinkxie]

please update the doc as well. thanks