Interpreter: cache getBshPrompt and ensure streams not null.

Cache the default prompt string retrieved via eval("getBshPrompt();"); for interactive mode, to avoid having to evaluate the string on every command.
Console ensures out and err streams are never null by returning default streams from System if not set.

src/main/java/bsh/Interpreter.java

@@ -170,6 +170,9 @@ public static void setShutdownOnExit(final boolean value) {
      */
     private boolean compatibility = COMPATIBIILTY;
 
+    /** Cached getBshPrompt for interactive mode. */
+    private String prompt = null;
+
     /* --- End instance data --- */
 
     /**
@@ -1168,13 +1171,15 @@ global namespace.  This may be from the getBshPrompt() command or may
         be defined by the user as with any other method.
         Defaults to "bsh % " if the method is not defined or there is an error.
     */
-    private String getBshPrompt()
-    {
+    private String getBshPrompt() {
+        if ( null != prompt )
+            return prompt;
         try {
-            return (String)eval("getBshPrompt()");
+            prompt = (String) eval("getBshPrompt()");
         } catch ( Exception e ) {
-            return "bsh % ";
+            prompt = "bsh % ";
         }
+        return prompt;
     }
 
     /**
@@ -1273,11 +1278,15 @@ public Reader getIn() {
 
         @Override
         public PrintStream getOut() {
+            if ( null == out )
+                out = System.out;
             return out;
         }
 
         @Override
         public PrintStream getErr() {
+            if ( null == err )
+                err = System.err;
             return err;
         }