Added --nostereo option

Ability to disable stereo panning effect (e.g., paint brush, UI
elements sound effect feedback, etc.), useful for users with
hearing impairment in one ear, or situations where one speaker or
headphone is being used.  Use "--nostereo" command-line option
or "nostereo=yes" config. file option.

docs/en/html/OPTIONS.html

@@ -16,10 +16,10 @@ version
 </h1>
 <h2>Options Documentation</h2>
 
-<p>Copyright (c) 2002-2018 by various contributors; see AUTHORS.txt<br/>
+<p>Copyright (c) 2002-2019 by various contributors; see AUTHORS.txt<br/>
 <a href="http://www.tuxpaint.org/">http://www.tuxpaint.org/</a></p>
 
-<p>December 18, 2018</p>
+<p>September 21, 2019</p>
 
 
 </center>
@@ -174,6 +174,12 @@ version
     cannot be used to reenable sounds if they were disabled using this option.)
   </dd>
 
+  <dt><code><b>nostereo=yes</b></code></dt>
+  <dd>
+    Disable stereo panning support. (Useful for users with hearing impairment
+    in one ear, or places where a single speaker or headphone is being used.)
+  </dd>
+
   <dt><code><b>noquit=yes</b></code></dt>
   <dd>
     <p>Disable the on-screen "Quit" button and prevent the <b>[Escape]</b> key
@@ -1512,6 +1518,7 @@ version
     --allowscreensaver<br>
     --startblank<br>
     --nosound<br>
+    --nostereo<br>
     --noquit<br>
     --noprint<br>
     --printdelay=<i>SECONDS</i><br>
@@ -1588,6 +1595,7 @@ version
     --disablescreensaver<br>
     --startlast<br>
     --sound<br>
+    --stereo<br>
     --quit<br>
     --print<br>
     --printdelay=0<br>