The 'time' esh plug-in
-----------------------

The 'time' plug-in provides basic functionality to record the time
it took a process to complete, similar to time(1)

It implements the 'process_raw_command' method. Here it strips away the 
"time" part of the original command so that the subsequent programs
can run normally. 

It also implements the 'process_built_in' method. Here it fetches the current
time and stores it in a variable called start

Finally, it uses the 'command_status_change' method to know when the 
status of a child process has changed. If a child terminates, it will fetch
the current time again and store it in a variable called stop. It will then
calculate the difference between start and stop to calculate the time that the
process took to complete. 

This plug-in supports jobs with multiple commands. It will print the time for
each command independently. 

This does not support background jobs. 

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Written by Lance Chao, Alice Wang Spring 2015 for CS 3214