Expose underlying tap-spec stream

Exposing the underlying tap-spec stream allows for using this sweet output when writing a custom reporter

Example:

``` js
var test = require('tape');
var tapSpec = require('tap-spec');

test.createStream()
  .pipe(tapSpec())
  .pipe(process.stdout);
```

What do you think about adding this? Let me know if there is anything you'd like changed. Happy to contribute.