Add getPeaks and cleanup methods

[esonderegger]

... as well as two example pages, demonstrating their usage.

This PR is intended to address the following issues:

• https://github.com/esonderegger/web-audio-peak-meter/issues/25
• https://github.com/esonderegger/web-audio-peak-meter/issues/42
• https://github.com/esonderegger/web-audio-peak-meter/issues/43