Make abbreviation optional

Make abbreviation optional

The ',.' global abbreviation replacement feature is meant.

It is still on by default, but can be disabled by default by setting
$NOABBREVIATION to something greater than zero in .zshenv; or temporarily by
hitting '^xA', which switches it off (another '^xA' will reenable it - that
way, we can still get it back on, if disabled by $NOABBREVIATION).

Note: Turning off abbreviation effectively disables the feature. The binding
      to ',.' still remains, though. Therefore, entering a comma (,) will
      still take a few moments (because zsh is waiting for a possible ',.').
      If you cannot be bothered to wait that long, quickly hit CTRL-V before
      hitting the comma key.
      I briefly looked for a better binding for the feature; but most (all?)
      of the easy-to-remember ones (like ALT-a, ALT-, or ALT-.) where already
      taken. Others didn't produce a useful character sequence on my keyboard
      (like CTRL-, or CTRL.); and I don't want '^xA' for it, because that
      takes four(!) key presses.
      If someone has a better solution, please say so!