Optionally disable increment for program counter

[purag]

Currently, branch instructions have to do something like this in their eval() function:

ProgramCounter.setValue(ProgramCounter.getValue() + offset);

archsim will still increment the value of the program counter, even if the user sets it. We should allow an "options" object as the second argument to setValue:

ProgramCounter.setValue(ProgramCounter.getValue() + offset, {
  increment: true
});

This gives user finer controls over their architecture, and means no nasty hacks to adjust for the mandatory increment.