By default, do not halt on BRK (#14)

This change introduces a new command line flag, '-b', that cause the
simulator to halt on the `BRK` instruction. By default, however, the
simulator will no longer halt on `BRK`.

As before, this behavior can be toggled in the preferences at run-time.

src/main/java/com/loomcom/symon/Preferences.java

@@ -29,8 +29,6 @@ public interface Preferences {
 
     int DEFAULT_PROGRAM_LOAD_ADDRESS = 0x0300;
 
-    boolean DEFAULT_HALT_ON_BREAK = true;
-
     JDialog getDialog();
 
     int getProgramStartAddress();