Add eachBuffer helper to RootView

This allows extensions to bind a callback to all current
and future buffers.

src/app/root-view.coffee

@@ -239,3 +239,10 @@ class RootView extends View
 
     @on 'editor-open', (e, editor) ->
       callback(editor)
+
+  eachBuffer: (callback) ->
+    for buffer in @project.getBuffers()
+      callback(buffer)
+
+    @project.on 'new-buffer', (buffer) ->
+      callback(buffer)

src/extensions/strip-trailing-whitespace.coffee

@@ -2,13 +2,7 @@ module.exports =
   name: "strip trailing whitespace"
 
   activate: (rootView) ->
-    for buffer in rootView.project.getBuffers()
-      @stripTrailingWhitespaceBeforeSave(buffer)
-
-    rootView.project.on 'new-buffer', (buffer) =>
-      @stripTrailingWhitespaceBeforeSave(buffer)
-
-  stripTrailingWhitespaceBeforeSave: (buffer) ->
-    buffer.on 'before-save', ->
-      buffer.scan /[ \t]+$/g, (match, range, { replace }) ->
-        replace('')
+    rootView.eachBuffer (buffer) ->
+      buffer.on 'before-save', ->
+        buffer.scan /[ \t]+$/g, (match, range, { replace }) ->
+          replace('')