Context filter tokens only support one token per filter

The token processing for context filters within the view field looks like a freeform text string, but it only supports either tokens, or text, but tokens within text, or multiple tokens per argument. This is undocumented, and fails gracefully without warning or error, making it very difficult to determine why things aren't working. Here are a couple of simple usecases:

1) An embedded view looks up two specific taxonomy terms that are itemized in specific separate fields. The view rendering them looks them up based on the + operator. The argument string would look something like this:
arg1/{{ fields.taxonomy1 }}+{{ fields.taxonomy2 }}/arg3

2) a global:null context filter exists to add some context from the parent view to some fields in an embedded display. The argument string for this could look like this:
{{ fields.nid }}/Row:{{ raw_fields.counter }}

These should be supported and currently aren't. This is resolved with the attached patch.

Hope this helps!