Use flags in command line input

[sl4ven]

Rather than passing input arguments in prescribed number and order, we should encourage students to use command line flags to specify input. For example, instead of giving example for launching area computation program as

$ ./compute_area 10 20

we should provide example that takes input arguments like this:

$ ./compute_area --width 10 --length 20

This way allows students to specify defaults for all input variables, and to enter input argument in any order at the command line.