create new labels feature for spectrogram plugin (#984)
create new labels feature for spectrogram plugin (#984)

* create new labels feature for spectrogram plugin

- create labels parameter (true|false)
- create labels element and canvas
- create labels frequency method loadLabels();
- update createWrapper() method and apply labels feature parameter.

* create new labels feature for spectrogram plugin

create new labels feature for spectrogram plugin

- create labels parameter (true|false)
- create labels element and canvas
- create labels frequency method loadLabels();
- update createWrapper() method and apply labels feature parameter.

* remove extra spaces

* removed one more extra white space :)

* update css formatting

* remove extra space

* remove extra space

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