add support for afero fs

[plastikfan]

see: afero

After some thought, I'm not sure that it is wise to introduce a dependency on afero, just for the niche requirement of supporting afero. The better solution would be to define a new repo (trafero or nefero), that provides the afero adapters that conform to the interfaces defined here in traverse. Any client requiring both afero and traverse support, would just use this new repo to provide adaptation, freeing up traverse to not have a non essential dependency.