/file: readDir to distinguish files from dirs

[mindrones]

It would be useful to be able to specify what we are looking for (files and/or dirs).

We could pass a second argument like below (non-breaking):

> readDir('aPath', 'dir')
['dir1', 'dir2']

> readDir('aPath', 'file')
['file.txt', 'file.csv']

> readDir('aPath')
['dir1', 'dir2', 'file.txt', 'file.csv']

The output could be partitioned (which would be a breaking change) like this:

> readDir('aPath')
{
  dir: ['dir1', 'dir2'],
  file: ['file.txt', 'file.csv']
}

or like this (breaking, less clear)

> readDir('aPath')
[
  ['dir1', 'dir2'],
  ['file.txt', 'file.csv']
]