'sum' Plug-in
-------------

'sum' is a built-in command and uses printf, so it needs SIGCHLD to be
blocked or else unintended behavior is possible.

'sum' takes in several numerical values and summerizes them into a single
integer value.


Sample usage with sample prompt
-------------------------------
esh> sum 1 2 3 4
10
esh> sum 10 20 30
60
esh> sum word
0
esh>


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Created by Brandon Gilliam (bgill93@vt.edu) and Ethan Groves (ethan01@vt.edu)
CS 3214 - Spring 2015