Assembler improvements:
* Diagnose duplicate labels
* Better diagnostic for jmp to invalid label
Examples:
* Added "3 levels of led brightness" from the micropython book
* Added "hello" example from pico-examples (turn an LED on and off while needlessly using PIO)
To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-pioasm`.
Read the [docs](http://circuitpython.readthedocs.io/projects/pioasm/en/latest/) for info on how to use it.