Use purescript-free and purescript-inject?

Would you take a PR to change the `MarkupM` monad to use `purescript-free`?

I would also like to be able to use `purescript-inject` to make the various HTML element smart constructors work with different free monads, so that we can extend the DSL in libraries.

I appreciate if that's not the goal of the library though, so I wanted to ask before starting to work on anything.