Keeping hvega and elm-vega in sync

[jwoLondon]

Firstly, great to see this hvega implementation!

I thought I should raise this general issue as a place to discuss how (or if) to keep hvega in sync with elm-vega.

In particular, I have recommended, from V.2.3, that the default style of elm-vega should use functions rather than type constructors for building a specification. Some details can be found in the 2.3.0 release notes. As Vega-Lite and Vega evolve, I plan to use only functions rather than types to support new functionality. This will make it easier to add new functionality without breaking changes and will significantly ease the introduction of full Vega support. I realise this may involve quite some work to refactor hvega to use this style, but I hope the effort would be worth it.