Add continuous example

[caternuson]

As requested in #13. Not sure if "continuous" just means printing values in a loop, which current example already does, or actually using the continuous mode of the VL53L0X.

This PR adds a basic example of using continuous mode. Looks like there might be various ways it could be done, based on API layout. But this is a simple looping example.

[ladyada]

i think this is the way to do it