[FEATURE] Immutable reference access to user data

**Is your feature request related to a problem? Please describe.** I have a few places where I need to read from the user data, then use something like `call_on_name`, but I can't use `user_data` or `with_user_data`, because I'll end up with 2 mutable borrows of cursive. **Describe the solution you'd like** Add methods to `Cursive` to allow accessing the user data immutably.