DirWalker
DirWalker lazily traverses one or more directory trees, depth first, returning successive file names.
Initialize the walker using
{:ok, walker} = DirWalker.start_link(path) # or [path, path...]
Then return the next n path names using
paths = DirWalker.next(walker [, n \\ 1])
Successive calls to next will return successive file names, until
all file names have been returned.