Display WDL Version/Feature Support

[patmagee]

I was looking through the miniwdl documentation and could not find which versions of WDL were currently supported. It would be great for the community to be able to quickly know which versions and which features of those versions (ie from a development spec) are currently supported. Tangential to this might be a centralized matrix for version/feature compatibility directly in the OpenWDL git repo itself, however that is a different discussion.