Restore previous selections after tailing newlines are added

Closes #496

src/packages/whitespace/lib/whitespace.coffee

@@ -1,11 +1,12 @@
 module.exports =
   activate: ->
-    rootView.eachBuffer (buffer) => @whitespaceBeforeSave(buffer)
+    rootView.eachEditSession (editSession) => @whitespaceBeforeSave(editSession)
 
   configDefaults:
     ensureSingleTrailingNewline: true
 
-  whitespaceBeforeSave: (buffer) ->
+  whitespaceBeforeSave: (editSession) ->
+    buffer = editSession.buffer
     buffer.on 'will-be-saved', ->
       buffer.transact ->
         buffer.scan /[ \t]+$/g, ({replace}) -> replace('')
@@ -16,4 +17,6 @@ module.exports =
             while row and buffer.lineForRow(row) is ''
               buffer.deleteRow(row--)
           else
+            selectedBufferRanges = editSession.getSelectedBufferRanges()
             buffer.append('\n')
+            editSession.setSelectedBufferRanges(selectedBufferRanges)

src/packages/whitespace/spec/whitespace-spec.coffee

@@ -79,3 +79,10 @@ describe "Whitespace", ->
       editor.insertText "no trailing newline"
       editor.getBuffer().save()
       expect(editor.getText()).toBe "no trailing newline"
+
+    it "does not move the cursor when the new line is added", ->
+      editor.insertText "foo"
+      expect(editor.getCursorBufferPosition()).toEqual([0,3])
+      editor.getBuffer().save()
+      expect(editor.getText()).toBe "foo\n"
+      expect(editor.getCursorBufferPosition()).toEqual([0,3])