x: The command line arguments, from x = commandArgs()
argument: The name of the argument.
prefix: The prefix to any argument of interest, the default is "--"
default: Default value to return is argument is missing, default to FALSE.
verbose: Boolean, if TRUE, shows a warning when the parameter is not found.
Returns
The value of the argument, assumed to be followed after '=' or, TRUE if nothing but the argument was found. If the argument is not found, FALSE is returned.