Better flow for renaming and deleting files

Previously when you delete or moved a file via
md or mm commands the NERDTree was asking you about what to do
with the remaining buffer of the just deleted or moved file. I
always press 'y' in this cases so I've decided to add a new parameter,
NERDTreeAutoDeleteBuffer which you can set to 1 in order to skip
this confirmation.

doc/NERD_tree.txt

@@ -658,6 +658,10 @@ NERD tree. These options should be set in your vimrc.
                                 Casade open while selected directory has only
                                 one child that also is a directory.
 
+|'NERDTreeAutoDeleteBuffer'|    Tells the NERD tree to automatically remove 
+                                a buffer when a file is being deleted or renamed
+                                via a context menu command.
+
 ------------------------------------------------------------------------------
 3.2. Customisation details                             *NERDTreeOptionDetails*
 
@@ -982,6 +986,20 @@ for Java projects.  Use one of the follow lines to set this option: >
     let NERDTreeCasadeOpenSingleChildDir=1
 <
 
+------------------------------------------------------------------------------
+                                          *'NERDTreeAutoDeleteBuffer'*
+Values: 0 or 1
+Default: 0.
+
+When using a context menu to delete or rename a file you may also want to delete
+the buffer which is no more valid. If the option is not set you will see a
+confirmation if you really want to delete an old buffer. If you always press 'y'
+then it worths to set this option to 1. Use one of the follow lines to set this
+option: >
+    let NERDTreeAutoDeleteBuffer=0
+    let NERDTreeAutoDeleteBuffer=1
+<
+
 ==============================================================================
 4. The NERD tree API                                             *NERDTreeAPI*