Add --backend-options argument

[lou1306]

Proposal

Add a --backend-options <opts> argument to SLiVER's CLI. If given, <opts> will be passed verbatim to the backend

Rationale

Mostly only for advanced users that know how to interact with the backend tool. Some specific use cases:

• When using CBMC, one may want to enable additional checks (e.g., --bounds-check to check array bounds) to debug the emulation program
• When using CADP, one may need a shortest counterexample for an unsafe system. To obtain it, they must pass -bfs to the backend