Treat priority as an arbitrary string

This matches what TaskWarrior does: priority is a UDA, and can be
redefined by the user's local config.

taskchampion/src/lib.rs

@@ -63,7 +63,7 @@ pub use errors::Error;
 pub use replica::Replica;
 pub use server::{Server, ServerConfig};
 pub use storage::StorageConfig;
-pub use task::{Annotation, Priority, Status, Tag, Task, TaskMut};
+pub use task::{Annotation, Status, Tag, Task, TaskMut};
 pub use workingset::WorkingSet;
 
 /// Re-exported type from the `uuid` crate, for ease of compatibility for consumers of this crate.