Fire event when initialization is done

Hi,
I added another user event to the initialization process, called `NERDTreePostSourceActions`.

#### Why is that useful (for me at least)?

I'm using the brillant [ctrlp](https://github.com/kien/ctrlp.vim) plugin in my daily workflow.
What I found was missing is that when I created a file/directory in `NERDTree` that file/dir was missing from the `ctrlp` index until I ran `:CtrlPClearCache`.

So I tried to add a `NERDTreeAddKeyMap` that calls a function which eg. adds a node via `a:node`. See [this issue](https://github.com/scrooloose/nerdtree/issues/98). This only works if you have a `.vim/nerdtree_plugins` folder.

My [vim_config](https://github.com/thenoseman/vim_config) is structured in a special way so I didn't like the necessity to have that folder. Call be pedantic :)

Now I can solve this via the new event.

#### A word of warning
I tried to follow the code- and documentation style I found in `NERDTree`but since I am a total n00b when it comes to vimscript _please_ do a thorough review of my changes.

Thanks for NERDTree!

--
Frank