fixed: directory navigation does not work on Cygwin

Directory tree navigation is broken because of directory signs which is
shown in UTF-8.

Cygwin is a Windows application, so it uses ASCII codepages and so
directory signs must be in ASCII, but if to modify
"nerdtree#runningWindows" function there are many other functions break
that convert paths, e.g.

So, the quick and reliable solution is to add a
separate function "nerdtree#runningCygwin" and use it in a specific
place.

autoload/nerdtree.vim

@@ -155,6 +155,11 @@ function! nerdtree#runningWindows()
     return has("win16") || has("win32") || has("win64")
 endfunction
 
+"FUNCTION: nerdtree#runningCygwin(dir) {{{2
+function! nerdtree#runningCygwin()
+    return has("win32unix")
+endfunction
+
 " SECTION: View Functions {{{1
 "============================================================