add two editor variables to line-numbers

this patch adds two editor variables to line-numbers, `line-format` and `current-line-display-function`.  this allows the user to change the width/format of the line numbers and to set the current-line display in relative numbers.

for example :
```
(setf (variable-value 'line-number-format :global) "~2D ")
(setf (variable-value 'current-line-display-function :global) (lambda () 0))
```

now the line-numbers will be narrower and the current-line will always be 0.

src/ext/line-numbers.lisp

@@ -24,7 +24,8 @@ two-character line-number column.")
 
 (define-editor-variable current-line-display-function
   (lambda () (line-number-at-point (current-point)))
-  "Set to desired current-line display when relative line numbers are active, for example, (lambda () 0) or (lambda () (string \" ->\")).")
+  "Set to desired current-line display when relative line numbers are
+active, for example, (lambda () 0) or (lambda () (string \" ->\")).")
 
 (define-attribute line-numbers-attribute
   (t :foreground :base07 :background :base01))