Add Editor::selectionFlashDuration rather than magic number

src/editor.coffee

@@ -146,6 +146,7 @@ class Editor extends Model
   selections: null
   suppressSelectionMerging: false
   updateBatchDepth: 0
+  selectionFlashDuration: 500
 
   @delegatesMethods 'suggestedIndentForBufferRow', 'autoIndentBufferRow', 'autoIndentBufferRows',
     'autoDecreaseIndentForBufferRow', 'toggleLineCommentForBufferRow', 'toggleLineCommentsForBufferRows',

src/selection.coffee

@@ -83,7 +83,7 @@ class Selection extends Model
       @cursor.needsAutoscroll = false if @needsAutoscroll?
       @marker.setBufferRange(bufferRange, options)
       @autoscroll() if @needsAutoscroll and @editor.manageScrollPosition
-      @decoration.flash('flash', 500) if needsFlash
+      @decoration.flash('flash', @editor.selectionFlashDuration) if needsFlash
 
   # Public: Returns the starting and ending buffer rows the selection is
   # highlighting.