Hide drawers in org-mode buffers using overlays.  These overlays
replace the visual display of drawers using their "display"
property.

Test out the functionality easily:
1. Open an org buffer.  (Indirect buffers are supported by
   org-hide-drawers.)
2. Enable `org-hide-drawers-mode'.  Drawers become "hidden": their
   displayed text is changed to `org-hide-drawers-display-string'.
   The actual text remains unchanged.
3. Un-hide drawers by calling `org-hide-drawers-toggle', or
   `org-hide-drawers-make-overlays'.
4. Hide drawers by calling `org-hide-drawers-toggle' again, or
   `org-hide-drawers-delete-overlays'.

Which drawers and property drawers are hidden and replaced by what
strings may be specified by the `org-hide-drawers-display-strings'
user option.  Org-hide-drawers supports matching against drawer
names, property drawer key names, and arbitrary user-defined
predicates.