ENH: Allow overriding sed command via TODOTXT_SED_COMMAND

On most Linux distributions (e.g. Ubuntu), there's no gsed command; the default "sed" is GNU sed with in-place editing capabilities.
For those systems, we don't want the redirect + file move workaround applied. However, detecting the sed capabilities is error-prone and just as inefficient as the separate "mv" it would avoid.
Instead, add a configuration that users can uncomment. Ideally, package maintainers for Linux distributions that have GNU sed ship with this already commented out in the default configuration.

Author: inkarkat

todo.cfg

@@ -101,3 +101,8 @@ export REPORT_FILE="$TODO_DIR/report.txt"
 # Set a default action for calling todo.sh without arguments.
 # Also allows for parameters for the action.
 # export TODOTXT_DEFAULT_ACTION=''
+
+## sed capabilities
+# 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

todo.sh

@@ -29,7 +29,9 @@ oneline_usage="$TODO_SH [-fhpantvV] [-d todo_config] action [task_number] [task_
 # Assumption: The in-place argument is the first
 # Assumption: Only a single file is processed with sed
 sed() {
-    if command -v gsed &>/dev/null; then
+    if [ -n "$TODOTXT_SED_COMMAND" ]; then
+        command "$TODOTXT_SED_COMMAND" "$@"
+    elif command -v gsed &>/dev/null; then
         gsed "$@"
     elif [ "$1" = '-i.bak' ]; then
         shift