Added SetComplexity to decoder

[avezina-ubik]

In order to support deep PLC, libopus added OPUS_SET_COMPLEXITY() to the decoder API.

This PR adds SetComplexity method to the decoder.

Note that if libopus < 1.5 this function will return ErrUnimplemented so users could easily ignore it like this:

dec, err := opus.NewDecoder(48000, 1)
if err != nil {
	// ... handle error
}

err = dec.SetComplexity(10)
if err != nil && err != opus.ErrUnimplemented {
	// ... handle error
}

[avezina-ubik]

This should help #54

[jackspirou]

@hraban curious if this can be merged?

[hraban]

Yes looks great sorry I forgot but it's on my radar will restore my go dev env in a bit