Support for afero.File.ReadFrom (to implement io.ReaderFrom)

`os.file` has the following function:

```
// ReadFrom implements io.ReaderFrom.
func (f *File) ReadFrom(r io.Reader) (n int64, err error) {
	if err := f.checkValid("write"); err != nil {
		return 0, err
	}
	n, handled, e := f.readFrom(r)
	if !handled {
		return genericReadFrom(f, r) // without wrapping
	}
	return n, f.wrapErr("write", e)
}
```

Is it something we can have in `afero`?