Optional channel encoding #182
Description
First of all, thanks for the library! It's a really neat idea.
I am wondering, is it possible to define optional encodings? Say for instance, you want to build a bar chart and optionally want to allow the encoding for error bars.
I believe encodable currently always requires a default encoding. Without it one runs into some errors. I think a way around it is by marking certain encodings as optional in the config type and setting the default value to something special but that feels very hacky. Is there another way to define an optional encoding or would you be interested in adding this feature in the future? I am happy to take a stab at it.