Read key = value pairs from command line and set entries in this dict. Array args is a list of the indices of command line arguments that we should look at, in order, if it's not given we look at all arguments. Returns false if fails, or if optional check that all mandatory values have been specified fails.
Dictionary of registered key/value pairs
Argument indices to use
Should we check if all mandatory parameters have been given
If true, ignore unknown keys in line