Add .rb extension to tempfile in vi mode

When you are in VI mode and you enter normal mode and press `v` it opens
up your `$EDITOR` to edit the current context. The issue is that your
`$EDITOR` does not know that you are editing Ruby code so you don't get
any syntax highlighting or formatting.

This change adds a `.rb` extension to the tempfile opened by the editor
which then ensures you get the correct syntax highlighting and
formatting.

lib/reline/line_editor.rb

@@ -2264,7 +2264,7 @@ def finish
   end
 
   private def vi_histedit(key)
-    path = Tempfile.open { |fp|
+    path = Tempfile.open(["reline", ".rb"]) { |fp|
       fp.write whole_lines.join("\n")
       fp.path
     }