add inc-fill-column and dec-fill-column

added next/previous-heading

added olivetti-mode (the function)

rename: s/window/buffer (but not always) this aims to solve (albeit only partially) the confusion between telescope' ``struct window'' and ncurses WINDOW. It also brings some more emacs-like nomenclature, which may be a good thing.

moved commands def to their own file and made the cmd_* functions public