Don't ignore 6000 and 6001

Add "interrupts enabled" flag
6000, 6001, 6002 should properly handle it
    6000 - skip next instruction if interrupts are enabled; disable interrupts (we should probably do this immediately)
    6001 - enable interrupts (after next instruction)
    6002 - disable interrupts (we should probably do this immediately)

Jules thinks clock is most likely interrupt source
    pdp8 source tries to map clock interrupts to "real time" - we might want to mangle that to make it deterministic

Sources of interrupts observed:
    PTR flag is set from loader, might be ignorable
    Memory management is poked from inside ISRs, might be ignorable (MMU should be optional)
    TTO ready for output interrupt - this looks like a very likely candidate
    TTI input ready interrupt - this looks like a very likely candidate
        Looks like these happen even when there is no input available - might be caused by the program itself doing something a little funny