Improve the display of conditionals with `converge graph`

[rebeccaskinner]

We need an effective way to visualize conditionals. In 0.3.0 the graphed conditionals showed expanded macros with long names. We'd like to update this in some way so that it's more readable and easy for users to see what predicates are going to cause which branches to be evaluated.

This is somewhat dependent on #427 since that issue may require some rework of the conditional internals.