Gracefully handle `--help` and other unknown options

[paulapatience]

Though SOLAR does not support GNU-style long options (with two leading hyphens), it should probably interpret them as unknown options rather than as non-option arguments:

$ ./solar --help
Display help for Problem "--help":

This problem id is invalid

Perhaps using an option-parsing library would be advisable.

[sebastienledigabel]

Thanks Paul. The "--" are now possible.