provide the generic input field formlet

HTML5 defines input types such as "email" or "date".
Provide a generic formlet so that we don't have to cover this
ever-growing set piece-by-piece.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
allow strings as formlet parameters

It allows programmer to ommit unnecessary double string<->bytes
conversion when repeating input from the user.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>