Split text sent to Tolk backend to account for some sort of length limit.

Tolk seems to fail on strings larger than 325 characters in length. Here we:
* Send any strings with 300 or fewer characters through directly.
* For larger strings, split on whitespace boundaries, then create and send buffers of 300 or fewer characters.

This may not handle internationalized text, and may not handle someone bombarding TTS with a giant word. PRs for either welcome.

src/backends/tolk.rs

@@ -28,7 +28,25 @@ impl Backend for Tolk {
 
     fn speak(&mut self, text: &str, interrupt: bool) -> Result<(), Error> {
         trace!("speak({}, {})", text, interrupt);
-        self.0.speak(text, interrupt);
+        const BUFFER_LENGTH: usize = 300;
+        if text.len() <= BUFFER_LENGTH {
+            self.0.speak(text, interrupt);
+        } else {
+            if interrupt {
+                self.stop()?;
+            }
+            let tokens = text.split_whitespace();
+            let mut buffer = String::new();
+            for token in tokens {
+                if buffer.len() + token.len() > BUFFER_LENGTH {
+                    self.0.speak(buffer, false);
+                    buffer = String::new();
+                } else {
+                    buffer.push_str(token);
+                    buffer.push(' ');
+                }
+            }
+        }
         Ok(())
     }