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

[andreas-kupries]

trafficstars

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.

[andreas-kupries]

Rebased code pushed

[msteveb]

I still need to convince myself that there is enough value-add in this feature. Since history, and to a lesser extent editing, don't make sense for hidden input I'm not certain there is enough value here for it to be part of linenoise.

I will think about it a bit more though.