Add invert and de-modulate functionality

I’ve modified the library slightly to add two features I needed: The
ability to disable the IR modulation signal, and the ability to invert
the IR signal output. The combination of these features enable
scenarios where you want to control a 3rd party device by wiring an
Arduino directly to the IR led input terminal, rather than having an IR
link.
Add demo on new functionality

This demo shows how the new functionality added here can be used - it
allows the user to interactively toggle IR signal modulation and
inversion using the serial connection.