New timeit feature

Added a new timeit feature to measure the execution time of a command. 

Added a new extra_options parameter to the Run command. The timeit feature is only available if extra_options is set to true. extra_options is False by default.

An example and explanation of the timeit feature can be found in the example_extra.py file

I chose to hide the timeit option by default to avoid clouding the real command options list for users who don't need it. In the future other extra options can be added - some ideas include repeating the command x number of times to do benchmarking, and do a random sleep before running a command to avoid server cron jobs from running the command at the exact same time.

I welcome feedback on whether these features are useful, and whether there are some better ways to organize them. Thanks