Open a mirrored NERDTree with correct width (#1177)

* Make sure a mirrored NERDTree is displayed at correct width.

* Remove references to unused variables.

b:NERDTreeOldWindowSize was referenced, but never set anywhere. No need
to keep it around.

* Refactor: Initialize variables a different way.

Using the get() function allows us to "let" variables more directly,
without using the execute command or requiring strings to be escaped.
This also eliminates the s:initVariable function. The new format is
shown below, and defines a default value if the user didn't provide a
value in the vimrc file.

  let g:var = get(g: 'var', 'default value')

* Update version number in change log.

* Fix alignment of `let g:var = get(g:, 'var', <default>)` statements.

lib/nerdtree/creator.vim

@@ -172,6 +172,7 @@ function! s:Creator.createMirror()
     let t:NERDTreeBufName = bufferName
     call self._createTreeWin()
     exec 'buffer ' .  bufferName
+    call b:NERDTree.ui.restoreScreenState()
     if !&hidden
         call b:NERDTree.render()
     endif