Display only directory not all files

[AkhilPatoliya]

I wanted to display only directory in my app but here all the files are display like .txt, .jpg , .dat but i would display only directories.

[alishari]

I added it in this commit: https://github.com/alishari/android-storage/commit/82cd5162982320392701337bbd2e441aba601508

I will make a pull request, but you can use my fork if you want it.