Polyglot Flex system

[Mightyjo]

Hi all.

I finally have a MVP of the language backend for Flex working. The demo is on my GitHub.

The skeletons need cleanup before I'm ready to send a PR.

Please take a look and let me know your thoughts on the approach.

I've started a constrictor pattern around the M4 dependency in this work, too. I have a plan to implement the old skeleton macros (e.g. %%ifdef, %%inheader) in as part of the output filter chain. But first I need a nap.