new: Add default priority variable TODOTXT_PRIORITY_ON_ADD. (#246)

Adds the config / env variable `TODOTXT_PRIORITY_ON_ADD`, which when set to one of the capital letters `A` to `Z` is given as a priority to a new task, if a priority is not given on command line.

tests/t1040-add-priority.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+test_description='test the priority on add feature'
+. ./test-lib.sh
+
+## Normal use case
+echo "export TODOTXT_PRIORITY_ON_ADD=A" >> todo.cfg
+
+test_todo_session 'config file priority' <<EOF
+>>> todo.sh add take out the trash
+1 (A) take out the trash
+TODO: 1 added.
+
+>>> todo.sh -p list
+1 (A) take out the trash
+--
+TODO: 1 of 1 tasks shown
+EOF
+
+## Wrong value in config var
+echo "export TODOTXT_PRIORITY_ON_ADD=1" >> todo.cfg
+
+test_todo_session 'config file wrong priority' <<EOF
+>>> todo.sh add fail to take out the trash
+=== 1
+TODOTXT_PRIORITY_ON_ADD should be a capital letter from A to Z (it is now "1").
+
+>>> todo.sh -p list
+=== 1
+TODOTXT_PRIORITY_ON_ADD should be a capital letter from A to Z (it is now "1").
+EOF
+
+test_done