A customizable mode-line for neotree

Hi Pei,

This PR adds a `defcustom` to specify which type of mode-line to display. I added a `neotree` type which display the current parent directory with
some additional info about it:

    [x/y] directory (D F)

where `x` is the index of the current node in `directory`, `y` the total number of nodes in `directory`, and `D` and `F` the number of directories and files in `directory`. 

![capture d ecran 2014-12-28 a 22 25 49](https://cloud.githubusercontent.com/assets/1243537/5566117/c1effcae-8ee7-11e4-9337-4adafe897147.png)