Circular symlink casues infinite loop

[airarrazaval]

As the title says, circular symbolic links causes an infinite loop. To replicate this issue you can do the following:

mkdir test
cd test
ln -s . symlink

Then in your script if you run the following:

const directoryTree = require('directory-tree');

directoryTree('/path/to/test/folder');

[mihneadb]

Fixed in #90. Thanks!