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.

plugin/NERD_tree.vim

@@ -48,6 +48,7 @@ call s:initVariable("g:NERDTreeAutoCenterThreshold", 3)
 call s:initVariable("g:NERDTreeCaseSensitiveSort", 0)
 call s:initVariable("g:NERDTreeNaturalSort", 0)
 call s:initVariable("g:NERDTreeSortHiddenFirst", 1)
+call s:initVariable("g:NERDTreeUseTCD", 0)
 call s:initVariable("g:NERDTreeChDirMode", 0)
 call s:initVariable("g:NERDTreeCreatePrefix", "silent")
 call s:initVariable("g:NERDTreeMinimalUI", 0)
@@ -183,6 +184,12 @@ if g:NERDTreeHijackNetrw
     augroup END
 endif
 
+if g:NERDTreeChDirMode == 3
+    augroup NERDTreeChDirOnTabSwitch
+        autocmd TabEnter * if g:NERDTree.ExistsForTab()|call g:NERDTree.ForCurrentTab().getRoot().path.changeToDir()|endif
+    augroup END
+endif
+
 " SECTION: Public API {{{1
 "============================================================
 function! NERDTreeAddMenuItem(options)