Element-wise `automatic` configuration

[filchristou]

As mentioned in #89 (comment), currently Makie.automatic applies holistically per attribute. However, sometimes it might be needed to leave the attribute behave mostly automatic but adjust a thingy or two manually.

Example

graphplot(graph; elabels_rotation = DefaultDict(automatic, Edge(1,2) => π/2)

should have all labels with the default rotation (like passing elabels_rotation=automatic)
but tweak label rotation for Edge(1,2) to be π/2

[hexaeder]

Hm maybe that already works if you use nothing instead of automatic?

graphplot(graph; elabels_rotation = DefaultDict(nothing, Edge(1,2) => π/2)

On first glance, it looks like we're handling nothing as automatic in prep_vertex_attributes, prep_edge_attributes and getattr. Of course, the inconsistency between nothing and automatic is not great and should be resolved.

Related question on performance of such containers on discourse: https://discourse.julialang.org/t/vectors-of-unions-between-concrete-type-and-singleton-type/101158