New feature: Support hidden input (for passwords and the like).

Controlled by internal global flag variable "is_hidden".
Added two new accessor functions to set and retrieve the flag's state.
(Avoided changing the signature of linenoise()).

Activating hidden input
- Causes all entered characters to be echoed back as '*'.
- Disables navigating and searching the history.
- Disables line completion.
- All other editing commands are left active.