Add input from a file also fixed NULL pointer in Char* Array

Added the run command which tells quash to use a .txt file in the
current directory and run each of its commands. Quash may now
also take in that file as an argument when you first run quash.
Example: ./quash input.txt will run quash then execute any
commands specified in input.txt.

Also fixed main.cpp's formation of the char * array as per Kulkarni's
email. Made sure that the last element of the array pointed to NULL