New cofiguration option: NERDTreeAlwaysSplit

Hi,

I've made some small changes to make NERDTree open every file in a new window. This way every file will stay opened until closed explicitly. If a file has already been opened in a window, this window will be brought to front instead of creating a new one. I'm using NERDTreeAlwaysSplit for about two years now and really like it, because it creates some kind of "tab-like" feeling.

As you can see in commit e77accd the changes in the code are really small and the always split functionality is disabled by default. In the two years of using my patch I never experienced any bugs or side-effects caused by my changes.

Hope hearing from you :)

Baibai
Torben