Allow priority to be specified in constraints

This extends the format to add an optional “@ priority” to the end, pulling the values as either doubles or values from the metrics dictionary.

I thought about allowing the default values of `UILayoutPriorityRequired`, `UILayoutPriorityDefaultHigh`, `UILayoutPriorityDefaultLow`, and `UILayoutPriorityFittingSizeLevel` as strings, but it seemed conceptually simpler to just require values as numbers or entries in the metrics dictionary (and matches Apple’s VFL spec).

(There’s also a fix for a existing typo in the readme in here)