Put in support for hidden input. Modified command loop, ensure non-hidden operation.
New hook for continuation line handling, robustness fix, tweak.

- Use of "info complete" is Tcl-specific.
  Replaced with a hook to check for complete line, using it as default.

- Moved setting regular input mode to just before the prompting, to
  prevent callbacks like line completion, continuation, etc. from
  messing with us.

- Calling the primitive prompter directly, no need to go through
  the user command.