Expose testem ci reporter option.

This came about as a way to more easily integrate lineman's testem output into my CI environment. This allows you to configure testem's ci reporter option and direct the output to an external file. In `applicaton.js` you can now do:

```javascript
{ 
  'spec-ci': {
    options: {
      reporter: '<whatever testem -R supports>'
      reporterOutput: '<path to test output file>
  }
}
```

I haven't done much Node development before, so any comments are appreciated. Assuming this change seems appropriate, if there are suggestions on testing, documentation, etc. please advise and I'll try to update accordingly. Thanks again for lineman! :)