Returns -1 for the priority of items that are not found in a Typeset's priority lookup.
Returns -1 for the priority of items that are not found in a Typeset's priority lookup.

This fixes a bug where attributes not attached to a format would break priority sorting for formats while rendering (undefined - 2 is NaN). This caused formatting elements to be broken up awkwardly even when formatting data was adjacent.

jacwright/typewriter,jacwright/typewriter