Fix #1080 -- Make `nikola init` ask for stuff.

This is #1080.

Note that this is *not done yet*.  There are a few more things we could
(and should) ask about, though that will require changes in conf.py.in.  If
anyone has any ideas, thoughts, nitpicks — I’ll happily listen.

(also, we could improve the languages part a little by allowing to list available locales.)