Expose a isInstalled() method

Author: gnodet

src/main/java/org/fusesource/jansi/AnsiConsole.java

@@ -285,6 +285,13 @@ synchronized static public void systemReinstall() {
         }
     }
 
+    /**
+     * check if the streams have been installed or not
+     */
+    synchronized public static boolean isInstalled() {
+        return installed > 0;
+    }
+
     /**
      * undo a previous {@link #systemInstall()}.  If {@link #systemInstall()} was called
      * multiple times, {@link #systemUninstall()} must be called the same number of times before