add two editor variables to line-numbers, line-number-formatand curre…

…nt-line-value

add two editor variables to line-numbers, `line-format` and `custom-current-line`. they allow the user to set the line format and choose a custom current line if they want.

for example :

`(setf (variable-value 'line-number-format :global) "~2D ")`

for a narrow line-number column. and

`(setf (variable-value 'custom-current-line :global) "->")`

for a current-line indicator of `->`

src/ext/line-numbers.lisp

@@ -57,10 +57,11 @@ With a positive universal argument, use relative line numbers. Also obey the glo
 (defun compute-line (buffer point)
   (if *relative-line*
       (let ((cursor-line (line-number-at-point (buffer-point buffer)))
-            (line (line-number-at-point point)))
+            (line (line-number-at-point point))
+            (custom-line (variable-value 'custom-current-line :default buffer)))
         (if (= cursor-line line)
-            (if (variable-value 'custom-current-line)
-                (variable-value 'custom-current-line :default buffer)
+            (if custom-line
+                custom-line
                 line)
             (abs (- cursor-line line))))
       (line-number-at-point point)))