Take position option for marker.
Take position option for marker.

Partially addresses #16. However, the preset markers must be added after
the audio is loaded and processed.

agamemnus/wavesurfer.js,ffxsam/wavesurfer.js,katspaugh/wavesurfer.js,ale-grosselle/wavesurfer.js,agamemnus/wavesurfer.js,ale-grosselle/wavesurfer.js,ffxsam/wavesurfer.js,katspaugh/wavesurfer.js