Added Execution Time

Added the ability to append the scripts execution time to the end of
the output, similar to how Sublime Text does it.

![screen shot 2014-11-12 at 4 41 25 pm](https://cloud.githubusercontent.com/assets/5246169/5019202/6405a876-6a8b-11e4-96d4-0b9e957828de.png)
![screen shot 2014-11-12 at 4 41 34 pm](https://cloud.githubusercontent.com/assets/5246169/5019203/6408c312-6a8b-11e4-80c3-9aff0651c982.png)