Add position to syntax

[thomasp85]

Supersedes #75

We should support a position parameter across all layers, with possible values being 'none', 'stack', 'dodge' at least for now. vegalite does stacking automatically right now but it interacts badly with #82 because we can no longer deduce the scale range.

My plan would be to calculate stack position directly in the data while letting the writer handle dodge since we don't have a concept of how incremental movement on a discrete scale would work. But since dodging doesn't affect the scale calculations it's fine to relegate it to the writer.

Since #82 is already turning out to be the mother of all PRs I'll not add this in that PR even though the PR will momentarily make stacking work bad