Add HIDE_CUSTOM_SUBSTITUTION for customization and add-ons.

Add-ons or users may want to hide parts of the task text from the output.
Though this can already be solved through TODOTXT_FINAL_FILTER, augmenting the configured value is not trivial, and it introduces another SED command into the already long _list() pipeline. Putting an additional HIDE_CUSTOM_SUBSTITUTION into the existing pipeline has hardly any performance implications, and makes the realization of this use case trivial.

tests/t1300-ls.sh

@@ -53,6 +53,18 @@ test_todo_session 'checking TODOTXT_FINAL_FILTER' <<EOF
 TODO: 3 of 3 tasks shown
 EOF
 
+#
+# check the custom hiding
+#
+test_todo_session 'checking HIDE_CUSTOM_SUBSTITUTION' <<EOF
+>>> HIDE_CUSTOM_SUBSTITUTION='[tT]h' todo.sh ls
+2 aaa zzz is line should be first.
+3 bbb yyy is line should be second.
+1 ccc xxx is line should be ird.
+--
+TODO: 3 of 3 tasks shown
+EOF
+
 #
 # check the filtering of TERM
 #

todo.sh

@@ -814,6 +814,7 @@ _list() {
         | sed '''
             s/'"${HIDE_PROJECTS_SUBSTITUTION:-^}"'//g
             s/'"${HIDE_CONTEXTS_SUBSTITUTION:-^}"'//g
+            s/'"${HIDE_CUSTOM_SUBSTITUTION:-^}"'//g
           '''                                                   \
         | eval ${TODOTXT_FINAL_FILTER}                          \
     )