Describe the bug
P (modify task priority) command doesn't take customized priority values into account: it only allows modifying priority by using h, m, l or n.

To Reproduce
Run test case to use uda.priority.values=A,B,C

1. Try to use P to change priority to C on task 1 => impossible (doesn't do anything)
2. Try to use P to change priority to h on task 1 => crash

Expected behavior

1. You should be able to set priority to C
2. Should be a no-op (ideally display an/Taskwarrior error message)

Priority (h,l,m,n) should not be shown but Priority (A,B,C) should be displayed instead (n [none] to reset priority field could probably be left out).
Alternatively, a mere Priority could be shown instead, provided autocompletion is available, as with modify task project command.

Test case
priority-alternate-values.sh.txt