plugins: document spectrogram labels option

example/spectrogram/app.js

@@ -13,7 +13,8 @@ document.addEventListener('DOMContentLoaded', function() {
         cursorColor: 'navy',
         plugins: [
             WaveSurfer.spectrogram.create({
-                container: '#wave-spectrogram'
+                container: '#wave-spectrogram',
+                labels: true
             })
         ]
     };

example/spectrogram/index.html

@@ -92,7 +92,8 @@ <h4>Quick Start</h4>
     plugins: [
         WaveSurfer.spectrogram.create{
             wavesurfer: wavesurfer,
-            container: "#wave-spectrogram"
+            container: "#wave-spectrogram",
+            labels: true
         })
     ]
 });
@@ -109,6 +110,7 @@ <h4>Options</h4>
                       <li><code>container</code> - <em>required</em> - the element in which to place the spectrogram, or a CSS selector to find it.</li>
                       <li><code>fftSamples</code> - number of FFT samples (<code>512</code> by default). Number of spectral lines and height of the spectrogram will be a half of this parameter.</li>
                       <li><code>frequenciesDataUrl</code> - URL to load spectral data from.</li>
+                      <li><code>labels</code> - Whether or not to display frequency labels.</li>
                     </ul>
                 </div>
             </div>

src/plugin/spectrogram.js

@@ -222,6 +222,7 @@ const FFT = function(bufferSize, sampleRate, windowFunc, alpha) {
  * which to render
  * @property {number} fftSamples=512 Number of samples to fetch to FFT. Must be
  * a power of 2.
+ * @property {boolean} labels Set to true to display frequency labels.
  * @property {number} noverlap Size of the overlapping window. Must be <
  * fftSamples. Auto deduced from canvas size by default.
  * @property {string} windowFunc='hann' The window function to be used. One of