ENH: Allow opt-in to sed emulation for add-ons via TODOTXT_SED_EXPORT_FOR_ADDONS flag

Most add-ons likely copied the "sed -i.bak ..." from todo.sh, but we can't be sure. Therefore, just document the issue in the configuration and offer a simple switch to enable it.
As most users are either on Linux or Windows/Cygwin where this isn't an issue, the rather technical language only applies to users of BSD or MacOS users who don't want to install GNU sed.

Author: inkarkat

todo.cfg

@@ -106,3 +106,8 @@ export REPORT_FILE="$TODO_DIR/report.txt"
 # If you have GNU sed or another sed that supports in-place editing
 # via -i[SUFFIX], uncomment this for a minuscule performance gain.
 # export TODOTXT_SED_COMMAND=sed
+
+# Uncomment this if you have add-ons that also do in-place editing
+# via sed, pass the -i.bak as the first argument like todo.sh itself,
+# and your system needs the in-place emulation.
+# export TODOTXT_SED_EXPORT_FOR_ADDONS=1

todo.sh

@@ -42,6 +42,8 @@ sed() {
         command sed "$@"
     fi
 }
+[ "$TODOTXT_SED_EXPORT_FOR_ADDONS" = 1 ] \
+    && export -f sed
 
 usage()
 {