Testling browser TAP reporter

We need a TAP reporter that runs in the browser to use nodeunit on [Testling](https://ci.testling.com/). I'm happy to whip one up, but I'm not sure where it belongs.

Options:
1. [karma-nodeunit](https://github.com/karma-runner/karma-nodeunit) is a separate repo. I could do the same for a TAP browser reporter. This direction feels a little funny because it's a generally useful reporter, like the [existing server TAP reporter](https://github.com/caolan/nodeunit/blob/master/lib/reporters/tap.js) and it'd be detached from nodeunit development. karma-nodeunit has the browser nodeunit build [committed](https://github.com/karma-runner/karma-nodeunit/blob/master/lib/nodeunit.js) in its source, which is a bit smelly.
2. Build it into nodeunit/dist/ with `make browser`. This direction feels funny because it's very much a second class citizen compared to the other reporters. Testling encourages test suites made of browserifiable CommonJS modules. There'd be no easy way to `npm install` and `require()` the reporter.

Thoughts?

首先阅读 lib/reporters/tap.js 中现有的服务器 TAP reporter，然后比较两个拟议的位置：单独的 karma-nodeunit repository，或通过 make browser 生成的 nodeunit/dist/。当确定放置位置，并且可以将浏览器 TAP reporter 与 Testling 及可 browserify 的 CommonJS 测试套件结合使用时，工作就完成了。

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