Fixes #13 by adding `--only-output-failures` flag.

This should do it. Here's how it looks on my setup.
```
☁  http-status-check [suppress-successful-output-flag] ⚡ node index.js
X koddsson.com https (https://koddsson.com) failed. Here are the problems:
  - code: ECONNREFUSED
  - errno: ECONNREFUSED
  - syscall: connect
_ LinkSilk WWW (http://www.linksilk.com) working as expected.
_ Google (http://google.com) testing disabled.
_ LinkSilk (http://linksilk.com) working as expected.
_ Missing URL example (http://www.guyellisrocks.com/2014/06/will-this-get-written.html) working as expected.
_ HTTP Status Check on Guy's Blog (http://www.guyellisrocks.com/2014/06/http-status-check.html) working as expected.
_ LinkSilk Redirect To 200 (http://www.linksilk.com) working as expected.
A total of 7 URIs were tested.
Failure count:  1
Success count:  5
Disable count:  1
☁  http-status-check [suppress-successful-output-flag] ⚡ node index.js --only-output-failures
X koddsson.com https (https://koddsson.com) failed. Here are the problems:
  - code: ECONNREFUSED
  - errno: ECONNREFUSED
  - syscall: connect
☁  http-status-check [suppress-successful-output-flag] ⚡
```