Support stack output

This adds support for `assert.ifError(e)` in tape.

Which outputs something like

```
not ok 3 Error: foo
  ---
    operator: error
    expected: undefined
    actual:   {}
    at: onUser (/home/raynos/projects/manga-feed/services/user/test/verify.js:33:20)
    stack:
      Error: foo
        at onUser (/home/raynos/projects/manga-feed/services/user/test/verify.js:33:28)
        at onUser (/home/raynos/projects/manga-feed/services/user/index.js:79:9)
        at /home/raynos/projects/manga-feed/node_modules/level-sublevel/shell.js:111:12
        at /home/raynos/projects/manga-feed/node_modules/level-sublevel/nut.js:121:19
        at Object._onImmediate (/home/raynos/projects/manga-feed/node_modules/level-mem/node_modules/memdown/memdown.js:125:5)
        at processImmediate [as _immediateCallback] (timers.js:330:15)
  ...
```

This pull request depends on #5

cc @scottcorgan