Major changes:
- add `move` and `grow` methods to move and grow (or shrink) windows
- fullscreen starts by centering the window, then proceeds through the
  sequence
Minor changes:
- update hs.grid when obj.GRID changes, making obj:init() unnecessary
- forget obj._pressed state after a second (with move & grow changes I
  found myself wanting to shift modifier keys without releasing them,
  this means I can just pause)
- refactor code to make it easier (for me) to understand
- tweak the documentation a little

handle repeated sizes in sizes and fullscreenSizes, 'c' sizes when there is nothing stored, and some other edge cases.

(as it does going or growing)

…` on direction change).