It seems like the make_constraint() function was designed to allow fully
defining a constraint with a single call, but currently when creating
Armature constraints, its targets have to be created separately after
the make_constraint() call.

This patch addresses this, allowing you to pass a "targets" dictionary.
Armature constraint targets only have three properties, which are "target",
"subtarget" and "weight". For convenience, since 99.99% of the times the
"target" will be the rig, that doesn't have to be specified, but it can be.

Differential Revision: https://developer.blender.org/D9092