ENH: Added highlighting of done (but not yet archived) tasks.

This makes done tasks more stand out (like prioritized tasks) in the task list. Default highlighting is LIGHT_GREY, which seems to be modest and go well with the assumed dark terminal background.
Reintroduced the OSX-compatible sed filter that was introduced by commit 8b7e2e6aad and superseded by the PRI_X coloring generalization.

todo.cfg

@@ -38,18 +38,26 @@ export TMP_FILE="$TODO_DIR/todo.tmp"
 # export WHITE='\\033[1;37m'
 # export DEFAULT='\\033[0m'
 
-# === PRIORITY COLORS ===
+# === COLORS ===
 
-## Priorities can be any upper-case letter.
-## A,B,C are highlighted; you can add coloring for more.
 ## Uncomment and edit to override these defaults.
-
+## Reference the constants from the color map above,
+## or use $NONE to disable highlighting.
+#
+# Priorities can be any upper-case letter.
+# A,B,C are highlighted; you can add coloring for more.
+#
 # export PRI_A=$YELLOW        # color for A priority
 # export PRI_B=$GREEN         # color for B priority
 # export PRI_C=$LIGHT_BLUE    # color for C priority
 # export PRI_D=...            # define your own
 # export PRI_X=$WHITE         # color unless explicitly defined
 
+# There is highlighting for tasks that have been done,
+# but haven't been archived yet.
+#
+# export COLOR_DONE=$LIGHT_GREY
+
 # === BEHAVIOR ===
 
 ## customize list output