Compatibility fix: rename --shell option for compatibility with the console in Symfony Framework

User interface BC break from 0.4.0: The --shell option introduced yesterday in 0.4.0 has been removed in favour of an argument to the existing --generate-hook option. This was required as the default console application in Symfony Framework has a global --shell option which conflicts with the one in this command, causing a logic exception. Forcing a shell type is now done as follows:

$ myprogram _completion --generate-hook zsh