replace: Completely merge given priority / date with existing

So that any combination of priority / date entered in the replacement will replace the corresponding original ones, but if they are left out, the original ones will be kept.
In essence, omitted stuff will be kept, added stuff will override, only deletion of existing stuff is not possible (but this is replace, after all).

Fixes #386

todo.sh

@@ -459,20 +459,31 @@ replaceOrPrepend()
 
   # Retrieve existing priority and prepended date
   local -r priAndDateExpr='^\((.) \)\{0,1\}\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} \)\{0,1\}'
-  priority=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\1/" "$TODO_FILE")
-  prepdate=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\2/" "$TODO_FILE")
-
-  if [ "$prepdate" ] && [ "$action" = "replace" ] && [ "$(echo "$input"|sed -e "s/${priAndDateExpr}.*/\\1\\2/")" ]; then
+  originalPriority=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\1/" "$TODO_FILE")
+  priority="$originalPriority"
+  originalPrepdate=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\2/" "$TODO_FILE")
+  prepdate="$originalPrepdate"
+  if [ "$action" = "replace" ]; then
+    replacementPrepdate="$(echo "$input"|sed -e "s/${priAndDateExpr}.*/\\2/")"
+    if [ "$replacementPrepdate" ]; then
       # If the replaced text starts with a [priority +] date, it will replace
       # the existing date, too.
-    prepdate=
+      prepdate="$replacementPrepdate"
+    fi
+    replacementPriority="$(echo "$input"|sed -e "s/${priAndDateExpr}.*/\\1/")"
+    if [ "$replacementPriority" ]; then
+      # If the replaced text starts with a priority, it will replace
+      # the existing priority, too.
+      priority="$replacementPriority"
+    fi
+    input="$(echo "$input"|sed -e "s/${priAndDateExpr}//")"
   fi
 
   # Temporarily remove any existing priority and prepended date, perform the
   # change (replace/prepend) and re-insert the existing priority and prepended
   # date again.
   cleaninput "for sed"
-  sed -i.bak -e "$item s/^${priority}${prepdate}//" -e "$item s|^.*|${priority}${prepdate}${input}${backref}|" "$TODO_FILE"
+  sed -i.bak -e "$item s/^${originalPriority}${originalPrepdate}//" -e "$item s|^.*|${priority}${prepdate}${input}${backref}|" "$TODO_FILE"
   if [ "$TODOTXT_VERBOSE" -gt 0 ]; then
     getNewtodo "$item"
     case "$action" in