Allow multiple lines in minibuffer.

I picked 17 lines allowed because that's what my emacs seems to take
until it starts to open a separate buffer. Perhaps a better heuristic
should be employed but this should be fine unless someone complains. We
could make it customisable.

Closes #477, fixes #478, mention in #485.
Don't use ‘undefined’ for IORef initialisation.

While technically we update it right after anyway, this makes me feel
safer in case any code ever moves around.
update to vty 5.1 interface
correct compile of Yi.Config.Simple when vty is not used
removed duplicate StandaloneDeriving