refactor: name diff orientations vertical and horizontal

src/ruby_config.rb

@@ -142,7 +142,7 @@ module Vamp
       @diff_context_lines = 10
       @diff_expand_lines = 15
       @diff_view_mode = "split"
-      @diff_split_orientation = "left_right"
+      @diff_split_orientation = "vertical"
     end
 
     # @param path [String] relative to the configuration file
@@ -215,12 +215,12 @@ module Vamp
       @diff_view_mode = mode
     end
 
-    # @param orientation [String] "left_right" or "above_below"
+    # @param orientation [String] "vertical" or "horizontal"
     # @return [String]
     def diff_split_orientation=(orientation)
       orientation = nonempty_string!(orientation, "diff split orientation")
-      unless %w[left_right above_below].include?(orientation)
-        raise ArgumentError, "diff split orientation must be left_right or above_below"
+      unless %w[vertical horizontal].include?(orientation)
+        raise ArgumentError, "diff split orientation must be vertical or horizontal"
       end
       @diff_split_orientation = orientation
     end