Add customizable behavior allowing backward-delete of blank sexps

This also changes the return value of `sp-point-in-blank-sexp` to
return the beginning and ending positions of the enclosing regexp.

smartparens.el

@@ -1535,6 +1535,12 @@ kill \"subwords\" when `subword-mode' is active."
  :type 'boolean
  :group 'smartparens)
 
+(defcustom sp-delete-blank-sexps nil
+ "If non-nil, allow deletion of enclosing pair when it only
+ contains whitespace."
+ :type 'boolean
+ :group 'smartparens)
+
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Selection mode handling
@@ -1829,14 +1835,18 @@ If optional argument P is present test this instead of point."
 
 If optional argument P is present test this instead of point.
 
+Returns a cons containing the position of the beginning and end
+delimiters.
+
 Warning: it is only safe to call this when point is inside a
 sexp, otherwise the call may be very slow."
  (save-excursion
  (when p (goto-char p))
  (-when-let (enc (sp-get-enclosing-sexp))
- (sp-get enc (string-match-p
- "\\`[ \t\n]*\\'"
- (buffer-substring-no-properties :beg-in :end-in))))))
+ (sp-get enc (when (string-match-p
+ "\\`[ \t\n]*\\'"
+ (buffer-substring-no-properties :beg-in :end-in))
+ (cons :beg :end))))))
 
 (defun sp-char-is-escaped-p (&optional point)
  "Test if the char at POINT is escaped or not.
@@ -9141,6 +9151,8 @@ If on a closing delimiter, move backward into balanced expression.
 
 If on a opening delimiter, refuse to delete unless the balanced
 expression is empty, in which case delete the entire expression.
+If `sp-delete-blank-sexps' is non-nil, the expression is also
+considered empty if it contains only white-space.
 
 If the delimiter does not form a balanced expression, it will be
 deleted normally.
@@ -9186,6 +9198,13 @@ Examples:
  ok)
  ;; make this customizable
  (setq n (1- n)))
+ ((and sp-delete-blank-sexps
+ (sp--looking-back (sp--get-opening-regexp (sp--get-pair-list-context 'navigate)))
+ (-when-let ((beg . end) (sp-point-in-blank-sexp))
+ (goto-char beg)
+ (delete-char (- end beg))
+ t))
+ (setq n (1- n)))
  ((and (sp-point-in-string)
  (save-excursion (backward-char) (not (sp-point-in-string))))
  (setq n 0))