Add TODOTXT_FINAL_FILTER to provide a final custom list filter.

The idea behind this is to allow a config file or addon to inject
a list output filter after all the other filtering has been
performed. The existing TODOTXT_SORT_COMMAND provides a hook
for a custom filter prior to colorization and priority/context/proj
hiding; this new TODOTXT_FINAL_FILTER provides a hook for custom
filtering after the colorization and pri/con/proj hiding.

(I found this hook was needed when I wanted to trim the output to a
maximum line length.)

Note: I've added a number of tests of the list command, including
one for this new variable, as part of this change.

Signed-off-by: Emil Sit <sit@emilsit.net>

todo.cfg

@@ -52,4 +52,15 @@ export TMP_FILE="$TODO_DIR/todo.tmp"
 # === BEHAVIOR ===
 
 ## customize list output
+#
+# TODOTXT_SORT_COMMAND will filter after line numbers are
+# inserted, but before colorization, and before hiding of
+# priority, context, and project.
+#
 # export TODOTXT_SORT_COMMAND='env LC_COLLATE=C sort -f -k2'
+
+# TODOTXT_FINAL_FILTER will filter list output after colorization,
+# priority hiding, context hiding, and project hiding. That is,
+# just before the list output is displayed.
+#
+# export TODOTXT_FINAL_FILTER='cat'