Add Shell::param()

Resolves https://github.com/cakephp/cakephp/issues/4249

I had the same issues with the cakephp/upgrade repo and params access in it. So I thought it should be fixed ASAP in order for me to continue there.

It is pretty much the same method as in Request::param().
Only difference: not exists => null (instead of false) returned. This makes more sense here IMO.