Allow to read current writer offset

[dim]

Something like this would be nice:

// Offset returns the latest Offset written (not necessarily synced).
func (wal *WAL) Offset() Offset {
	wal.mx.RLock()
	off := newOffset(wal.fileSequence, wal.position)
	wal.mx.RUnlock()

	return off
}

[dim]

Actually, it would probably make much more sense to extend Write to Write(... []bufs) (int, Offset, error)