feat(ambient sensor)

This adds #25 (closed) , still a few things where I'm not sure how we'd want to do it:

• get_reading() should imo throw an exception if continuous reading isn't active, but I'm not sure how I'd want to achieve that. Through checking a separate epic_ambient_is_running() or by having epic_ambient_get() return some magic number indicating an error?
• Similarly, do we want start() and stop() to throw an exception if the continuous mode isn't running?

Also, maybe it'd be nicer to move the continuous reading stuff to some other module in the future to (1) make it reusable and (2) nicely read multiple adc channels in one go.