new options and defaults for "js-test"

@kevinranks This PR implements all changes requested in https://github.com/Ranks/emojione/pull/9 :)

What's new:

  * now the script has a proper help displayed on the command line when the user types ```js-test --help``` (or any invalid parameter)
  * opening the browser with the tests is **disabled** by default. User may still do this with ```js-test --run```
  * default target file "tests.html" can now be overwritten with ```js-test --output=/some/other/file.html``` 

Again, if you bump into any issues, please let me know! Thanks again for Emojione, I love it :D