Add 'scrollbarchar' option (#2342)

This lets you specify a character that will get used for rendering the
scrollbar.

Co-authored-by: Zachary Yedidia <zyedidia@gmail.com>

runtime/help/options.md

@@ -311,6 +311,10 @@ Here are the available options:
 
     default value: `false`
 
+* `scrollbarchar`: specifies the character used for displaying the scrollbar
+
+    default value: `|`
+
 * `scrollmargin`: margin at which the view starts scrolling when the cursor
    approaches the edge of the view.