prompt: add literal mode

[askalt]

Ctrl-v must tell a prompt to not interpret the following character. Press Ctrl-v and Enter. ^M must be printed. Ctrl-Home print the following: ?[1;5H, but it must work only if Ctrl-v was pressed previously (Ctrl-v, Ctrl-home) and print: ^[[1;5H.

So, need to add a literal mode activated by Ctrl-v, which allows not to interpret special keys. And, if this mode is not enabled, need to interpret them correctly.