Allow fine grain filter of git status

Many times you want to filter git status from staged/unstaged changes. As a graphic view for your `git add --patch` workflow.

Maybe this shortcuts could work
- `fu` only unstaged
- `fs` only staged
- `fg` show staged/unstaged files

as this seems for advanced users, activate through a global option.

```
g:NERDTreeAdvancedGitFilter
```
