Add reluctant joining

[anlsh]

Hi, I was a little annoyed by how hungry delete would often join words when I really just wanted to delete some extra whitespace between them.

;; State A
(defun foo ()    (bar))
                 ^
;; Press backspace
;; State B
(defun foo ()(bar))
             ^

I feel that better behavior for hungry deletion in state A would be to transition to the following state

;; State C
(defun foo () (bar))
              ^

From where the user could press backspace again to get to state C

This change adds a switch, hungry-delete-join-reluctantly, to enable the above behavior. The switch is off by default of course, so the default behavior should be unchanged

From my rudimentary testing, it seems to work as I expect it to. Let me know if the code looks good and if you'd consider merging the PR. If so, I'll amend the readme to add some documentation as well

[anlsh]

Looks like I should be defining the switch via defcustom at the very least