Separate html "shell" javascript; player now standalone

Closes #3.

The code which defines what the buttons do and downloads songs and so
forth are now completely separated from the player, which has the
following API:

 - `XMPlayer.initAudio` -> starts up audio context; it's available as
   `XMPlayer.audioctx`
 - `XMPlayer.loadXM(ArrayBuffer)` -> returns `true` if loaded, otherwise
   barfs randomly
 - `XMPlayer.playXM()` -> starts playing
 - `XMPlayer.pauseXM()` -> obvious
 - `XMPlayer.stopXM()` -> obvious; call this before loading a new one

Loading trackview.js is now optional; without it, the player won't do
any visualizations. Or, you can override the following to get callbacks:

 - `XMView.pushEvent(e)` -> push an audio event onto the queue. Called
   once per tick (about 50Hz, controlled by song). `e` contains fields:
   - `t` - audio timestamp
   - `vu` - Float32Array of RMS power (volume) for each channel
   - `scopes` - [Float32Array] of oscilloscope data, one array per
     channel; `XMView.scope_width` contains # of samples to produce here
   - `songpos` - position in the song (# patterns played)
   - `pat` - pattern number currently playing
   - `row` - row within pattern
 - `XMView.pause()` - pause visualization
 - `XMView.stop()` - stop/reset visualization

index.html

@@ -4,6 +4,7 @@
   <script src="xmeffects.js"></script>
   <script src="trackview.js"></script>
   <script src="xm.js"></script>
+  <script src="shell.js"></script>
   <style>
 
 .centered {