Adds an option to enable adaptive-last-n.

config.js

     enableRtpStats: true, // Enables RTP stats processing
     openSctp: true, // Toggle to enable/disable SCTP channels
     channelLastN: -1, // The default value of the channel attribute last-n.
+    adaptiveLastN: false,
     useRtcpMux: true,
     useBundle: true,
     enableRecording: false,

libs/colibri/colibri.focus.js

         else
         {
             elemName = 'channel';
-            if (('video' === name) && (self.channelLastN >= 0))
-                elemAttrs['last-n'] = self.channelLastN;
+            if ('video' === name) {
+                if (self.channelLastN >= 0) {
+                    elemAttrs['last-n'] = self.channelLastN;
+                }
+                if (config.adaptiveLastN) {
+                    elemAttrs['adaptive-last-n'] = 'true';
+                }
+            }
         }
 
         elem.c('content', { name: name });
         else
         {
             elemName = 'channel';
-            if (('video' === name) && (self.channelLastN >= 0))
-                elemAttrs['last-n'] = self.channelLastN;
+            if ('video' === name) {
+                if (self.channelLastN >= 0) {
+                    elemAttrs['last-n'] = self.channelLastN;
+                }
+                if (config.adaptiveLastN) {
+                    elemAttrs['adaptive-last-n'] = 'true';
+                }
+            }
         }
 
         elem.c('content', { name: name });