Set filetype unconditionally and after defining mappings.

Because NERD_Tree sets the filetype in its buffer, it's tempting to hook into the FileType event in order to customize it. Unfortunately, the buffer-local mappings are only defined _after_ setting the filetype, so one cannot un-/redefine them via this mechanism. (I know there are config variables and an API for that, but one may have other unforeseen uses.)
Therefore, swap the order and set the filetype last. Also, use :setlocal filetype= instead of :setfiletype. In the (remote) chance that a custom filetype detection mistakenly sets a filetype for the NERD_Tree buffer, the :setfiletype would be without effect, but we want to force the filetype (for the special syntax) here.
Fix typo Casade to Cascade
Add correspoding close action to cascade open single child dir
Fixed typo in NERD_tree.txt
Ugly hack to fix Error 121 when NERDTree is the active window and clicking on the command line below another window.
Fix typo
Issue 315 - Incorrect buffer deleted on file delete
Merge pull request #316 from ZeroProbability/master

Issue 315 - Incorrect buffer deleted on file delete
Merge pull request #299 from tophsic/master

Fix typo
Merge pull request #266 from staeff/patch-1

Fixed typo in NERD_tree.txt
Merge pull request #251 from pendulm/master

fix typo and smart close single child directory
update the NERDTreeCascadeOpenSingleChildDir doc
Merge branch 'better-set-filetype' of https://github.com/inkarkat/nerdtree into inkarkat-better-set-filetype
Merge branch 'inkarkat-better-set-filetype'
Merge branch 'master' of https://github.com/scrooloose/nerdtree into feature/merge-upsteam
Merge branch 'feature/merge-upsteam' into develop
update code in readme to auto-open a tree on startup

Closes #242
update the doc/quickhelp for :Bookmark

Make it clearer that the argument is optional.

Closes #229
update symlink highlighting

Previously we highlighted symlinks as one item (NERDTreeLink):

    symlinked_file -> /path/to/target

Split this out into 3 highlight groups:
    * NERDTreeLinkFile
    * NERDTreeLinkDir
    * NERDTreeLinkTarget

So we have:

    symlinked_dir/ -> /foo/bar
    -------------- ***********
         ^               ^
         |               |
    NERDTreeLinkDir   NERDTreeLinkTarget

Similarly for file links - with NERDTreeLinkFile instead of
NERDTreeLinkDir.

This allows users to modify how symlinks are highlighted. E.g. to make
them appear as normal files/dirs they could add this to their vimrc:

    hi link NERDTreeLinkFile NERDTreeFile
    hi link NERDTreeLinkDir NERDTreeDir
    hi link NERDTreeLinkTarget ignore
separate out syntax matching for Dir Arrows vs old style