got something working with seek table and fixed block sized flac

TODO: handle cases where sample is invalid:

whence == io.SeekEnd && sample is not negative
whence == io.SeekStart && sample is negative
whence == io.SeekCurrent and sample + current sample < 0 or > stream.Info.NSamples

This commit changes the way Stream is initialized quite a bit
and maybe it is less readable?  I personally think it's more readable
because I really like the variadic argument pattern so I've used it
quite a bit.

I think it addresses some of the concerns:

1:  Use *bufio.Reader as the default in order to match the current
    behaivor by calling

    stream, err = flac.New(r)

2:  Make Seek explicitly optional by calling:

    stream, err = flac.New(r, flac.EnableSeek)