Introducing bash-parser dependency

This is an investigatory PR to study the introduction of a POSIX compliant shell code parser using bash-parser.

I studied the cash workflow, and I think the majority of changes in source code would concern Vorpal.

They could be implemented directly there (in vorpal core) or we could patch vorpal and implement it only for cash.

We're tearing apart Vorpal right now anyway, so I think it would be a good idea to play with i there.