WIP: update Vega-Lite to version 2.5

[jakevdp]

A few tasks before merging

• wrap new field predicates (actually, field predicates are not wrapped at the moment; see Better access to logical operand predicates #695)
• handle .dsv data format
• deal with SortField backward incompatibility somehow?
• update docs on above features

[jakevdp]

I can't think of any good way to deal with the SortField change. If SortField were now not used in the schema, we could alias it to EncodingSortField so that old code will continue working. But since SortField still exists but refers to something different, that will serve to create confusion in the case that someone wants to use the new version of SortField.

I think we just need to stomach the API breakage and move on, though it will cause issues for some people when upgrading from 2.1 to 2.2.

I know @palewire has used SortField before, and others probably have too...

[kanitw]

We could think of another name and rename SortField in VL.

Ill be out of town for a week though so might nit be able to change that until i cole back

[palewire]

At this stage, I'm okay with eating API changes. I just wouldn't want to lose the functionality. What is it changing to?

[jakevdp]

OK, I think we'll just swallow the SortField -> EncodingSortField change at this point, then.