invalid bookmarks now placed after a blank line at the bottom of the
bookmarks file

if invalid bookmarks are detected, an error is outputted which directs
the user to :help NERDTreeInvalidBookmarks, which had been added to the
help file in this commit

we dont need it to default to 1 anymore as that was a hack for windows,
but now that we handle windows paths properly we dont need it

when the user hits B and opens the bookmarks table, stick the cursor at
the top of it

make the t/T on directory nodes open a fresh NERD tree for the selected
dir in a new tab, rather than a netrw.

when opening a file in a new split (with the <tab> map) we were doing
the :split with a "silent" modifier. This meant that a swap existed and
input was required from the user, the script would just freeze up

We needed to change how the bookmark name caching was done to work with
the new path-display-string caching. It has now been merged into path
string caching.

* Add a doc section for the NERDTreeQuitOnOpen option.
  * Update changelog.
  * update credits.

previoulsy, <c-k>/<c-j> did magic stuff if the selected node was a file.
i.e. jumped to the parent/parents next sibling. I chose to remove this
behaviour as it is more confusing than helpful.

adds :NERDTreeClose command and sets the filetype for the tree buffer to
"nerdtree"

dont warn windows users about setting NERDTreeChDirMode as drives are
handled properly now