Document the new preload metadata option

Author: goldfire

README.md

@@ -192,8 +192,8 @@ The volume of the specific track, from `0.0` to `1.0`.
 Set to `true` to force HTML5 Audio. This should be used for large audio files so that you don't have to wait for the full file to be downloaded and decoded before playing.
 #### loop `Boolean` `false`
 Set to `true` to automatically loop the sound forever.
-#### preload `Boolean` `true`
-Automatically begin downloading the audio file when the `Howl` is defined.
+#### preload `Boolean|String` `true`
+Automatically begin downloading the audio file when the `Howl` is defined. If using HTML5 Audio, you can set this to `'metadata'` to only preload the file's metadata (to get its duration without download the entire file, for example). 
 #### autoplay `Boolean` `false`
 Set to `true` to automatically start playback when sound is loaded.
 #### mute `Boolean` `false`

src/howler.core.js

@@ -572,7 +572,7 @@
       self._muted = o.mute || false;
       self._loop = o.loop || false;
       self._pool = o.pool || 5;
-      self._preload = (typeof o.preload === 'boolean' || typeof o.preload === 'string') ? o.preload : 'none';
+      self._preload = (typeof o.preload === 'boolean' || o.preload === 'metadata') ? o.preload : true;
       self._rate = o.rate || 1;
       self._sprite = o.sprite || {};
       self._src = (typeof o.src !== 'string') ? o.src : [o.src];
@@ -626,7 +626,7 @@
       }
 
       // Load the source file unless otherwise specified.
-      if (self._preload === true || self._preload === 'auto' || self._preload === 'metadata') {
+      if (self._preload && self._preload !== 'none') {
         self.load();
       }