m-audio loop-duration (#174)

[MarcusLongmuir]

Resolves #174.

This PR:

• adds a loop-duration attribute to m-audio

  The duration in seconds of the audio loop if loop is true. Can be shorter or longer than the audio file duration. Durations longer than the audio file will add silence. If not specified, the entire audio file will loop.

• refactors the m-audio audio playback to use the WebAudio API rather than an <audio> tag. This was necessary to allow precise timing of (re)starting playback when the loop duration was longer than the provided file.

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