Add option to exclude hidden files in 'ls' method

## Suggestion:

I think its very useful when you want to exclude the hidden files when listing a directory content.

Eg:

````bash
my_folder
├── .gitignore
├── README.md
├── package.json
├── public
│   ├── app.css
````
You could pass a parameter to exclude the hidden files:

``` coffeescript
fsu = require "fs-util"

fsu.ls "path/to/my_folder", true 

#returns '['path/to/my_folder/README.md', 'path/to/my_folder/package.json', 'path/to/my_folder/public' ]'

#Doesn't include the ".gitignore" file.
```