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.

todo.sh

@@ -821,6 +821,7 @@ _list() {
         | sed '''
             s/'"${HIDE_PROJECTS_SUBSTITUTION:-^}"'//g
             s/'"${HIDE_CONTEXTS_SUBSTITUTION:-^}"'//g
+            s/'"${HIDE_CUSTOM_SUBSTITUTION:-^}"'//g
           '''                                                   \
         | eval ${TODOTXT_FINAL_FILTER}                          \
     )