Add Loop2 that supports start and end positions

[MarkKremer]

This PR introduces Loop2 and deprecates Loop. Loop2 has options to configure a portion of the source streamer that must be looped. The new LoopTimes() is similar to the count argument of Loop but is given the number of repeats instead of the total number of plays.

The change from total play count to number of repeats is made partially to avoid the edge case of when a loop must be played 0 times. Because then it's unclear if the parts before and after the loop section must still be played. Number of repeats of 0 simply means play everything once.

The people over at the IKEMEN-Go project have their own implementation of this & I can see that it could be useful for other people as well.

Usage:

// Play streamer and repeat the section between the 5th and 20th second 2 times (played 3 times in total).
beep.Loop2(streamer, beep.LoopTimes(2), beep.LoopBetween(sampleRate.N(time.Second * 5), sampleRate.N(time.Second * 20)))