Adding an --optimize flag option

[Halbaroth]

This flag should behave as follow:

• In presence of the flag, the custom statements maximize, minimize and get-objectives should be available as SMT-LIB statements and Alt-Ergo may perform extra works to fulfill these objectives. In the current implementation, the FunSAT solver doesn't support optimization and we don't plan to support it in future. Thus, in the presence of --optimize, the binary should select the CDCL-Tableaux solver and produce a warning in presence of --sat-solver Tableaux or --sat-solver Tableaux-CDCL options.
• In absence of the flag, the name maximize and minimize are not reserved and get-objectives should produce a SMT-LIB error.