Colorization - color.recurring

- Added support for "color.recurring" configuration variable to
  colorize recurring tasks.
- Updated docs.

src/Config.cpp

@@ -144,6 +144,7 @@ void Config::createDefault (const std::string& home)
         fprintf (out, "#color.tag.bug=yellow\n");
         fprintf (out, "#color.project.garden=on_green\n");
         fprintf (out, "#color.keyword.car=on_blue\n");
+        fprintf (out, "#color.recurring=on_red\n");
         fprintf (out, "#shadow.file=%s/shadow.txt\n", dataDir.c_str ());
         fprintf (out, "#shadow.command=list\n");
         fprintf (out, "#shadow.notify=on\n");