Implement basics of #144 (watchdog)

One thing that I'm interested in is if this is required on k20-based boards different than Teensy. Without `watchdog::init(watchdog::Disabled)` the blink example will hang/missbehave if the values of `wait(...)` are bigger than let's say 1000. For values around `10` the fact that board is in reset loop is easy to overlook. Can anyone try this?

Other than this, as this is my first PR, please be gentle. :)
As far as I know `ioregs!` couldn't do anything about this as it would have to add the attribute to its enclosing scope. One option here would be to make `ioregs!` itself emit the `mod reg` block, although this is a bit more opinionated than I originally intended the macro to be.