Add GetReader

[tri-adam]

(*Descriptor).GetReadSeeker currently returns an io.ReadSeeker, but I cannot find any usage of it that would not be satisfied by the simpler io.Reader interface. In the interest of simplifying the underlying implementation in a major version bump, I think we should consider deprecating GetReadSeeker and introducing:

func (d *Descriptor) GetReader(fimg *FileImage) io.Reader

This change can be introduced in a minor version, which will provide time before the planned removal of GetReadSeeker in a v2 API.