New 'ids' command, and stdin reading

- New 'ids' command that returns a filtered set of task ID numbers, instead
  of the actual tasks.  For advanced pipeline use.
- Now supplements the command line with data read from standard input, which
  allows commands like:  echo 'add Pay the bills' | task

src/main.h

@@ -84,6 +84,7 @@ int handleAnnotate (std::string&);
 int handleDenotate (std::string&);
 int handleDuplicate (std::string&);
 int handleCount (std::string&);
+int handleIds (std::string&);
 void handleUndo ();
 void handleMerge (std::string&);
 void handlePush (std::string&);