Add lateral shell movement; decrease screen DELAY.

Add:
	src/screen.h
		-decrease QUIT_DELAY time, from 2000 to 400, with the logic that if
		a pause before the window quits is required an explicit (u)sleep() call
		will be made.

	src/interpreter/shell.h
		-add lateral movement across the command line
		-mod insertChar() and deleteChar() to slide forward/backward
		respectively the string following the cursor position, to account for
		the ability to move one's cursor.

Fix:
	src/interpreter/interpreter.c
		-sscanf() return value was misinterpreted (EOF is sent only in the case
		of a premature exit)-- fix value checking to ensure that all necessary
		values are properly scanned in from an arguments string.