add support for playing silence in say verb

lib/tasks/say.js

@@ -43,6 +43,9 @@ class TaskSay extends Task {
       // synthesize all of the text elements
       let lastUpdated = false;
       const filepath = (await Promise.all(this.text.map(async(text) => {
+        if (text.startsWith('silence://')) {
+          return `silence_stream://${text.slice(10)}`;
+        }
         const {filePath, servedFromCache} = await synthAudio(stats, {
           text,
           vendor,