Add support for Readdir in File interface

[c4milo]

I'm trying to use go-git for a tool but I'm getting stuck with the lack of interoperability of go-billy. Particularly, I need to cast billy.File to another custom File interface we have that implements Readdir but billy.File does not. How reasonable is to get this added to go-billy?

Thanks!

[c4milo]

I sent #6 addressing this, I hope it makes sense for you to add it.