Support tab-specific CWDs (#1032)

* Change CWD when switching tabs to the tab's NERDTree root.

* Remove commented-out code.

* List the new possible value for NERDTreeChDirMode in doc.

* Add new option to select between `:cd` and `:tcd`.

* Document the new NERDTreeUseTCD option.

* Update version number in change log.

lib/nerdtree/nerdtree.vim

@@ -20,7 +20,7 @@ function! s:NERDTree.changeRoot(node)
     call self.root.open()
 
     "change dir to the dir of the new root if instructed to
-    if g:NERDTreeChDirMode ==# 2
+    if g:NERDTreeChDirMode >= 2
         call self.root.path.changeToDir()
     endif