Refactoring: Extract the many complex sed arguments into sedTransformations array and move the comments closer (#273)

There's too much distance between the comments that describe the various transformations that are done on the referenced task, and the corresponding sed expressions.
By using a local Bash array, we can collect the arguments and have the comments close by.

Co-authored-by: Ali Karbassi <ali@karbassi.com>

todo_completion

@@ -48,23 +48,28 @@ _todo()
                         completions=$(eval 'TODOTXT_VERBOSE=0 TODOTXT_SOURCEVAR=\$DONE_FILE' $_todo_sh command listcon 2>/dev/null)
                         ;;
                     *)  if [[ "$cur" =~ ^[0-9]+$ ]]; then
+                            declare -a sedTransformations=(
                                 # Remove the (padded) task number; we prepend the
                                 # user-provided $cur instead.
+                                -e 's/^ *[0-9]\{1,\} //'
                                 # Remove the timestamp prepended by the -t option,
-                            # and the done date (for done tasks); there's no
-                            # todo.txt option for that yet.
-                            # But keep priority and "x"; they're short and may
-                            # provide useful context.
+                                # but keep any priority (as it's short and may
+                                # provide useful context).
+                                -e 's/^\((.) \)\{0,1\}[0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} /\1/'
+                                # Remove the done date and (if there) the timestamp.
+                                # Keep the "x" (as it's short and may provide useful
+                                # context)
+                                -e 's/^\([xX] \)\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} \)\{1,2\}/\1/'
                                 # Remove any trailing whitespace; the Bash
                                 # completion inserts a trailing space itself.
+                                -e 's/[[:space:]]*$//'
                                 # Finally, limit the output to a single line just as
                                 # a safety check of the ls action output.
+                                -e '1q'
+                            )
                             local todo=$( \
                                 eval TODOTXT_VERBOSE=0 $_todo_sh '-@ -+ -p -x command ls "^ *${cur} "' 2>/dev/null | \
-                                sed -e 's/^ *[0-9]\{1,\} //' -e 's/^\((.) \)\{0,1\}[0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} /\1/' \
-                                    -e 's/^\([xX] \)\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} \)\{1,2\}/\1/' \
-                                    -e 's/[[:space:]]*$//' \
-                                    -e '1q' \
+                                sed "${sedTransformations[@]}" \
                             )
                             # Append task text as a shell comment. This
                             # completion can be a safety check before a