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/util.h

@@ -72,6 +72,7 @@ bool taskDiff (const Task&, const Task&);
 std::string taskDifferences (const Task&, const Task&);
 std::string taskInfoDifferences (const Task&, const Task&);
 std::string renderAttribute (const std::string&, const std::string&);
+std::string compressIds (const std::vector <int>&);
 
 #endif
 ////////////////////////////////////////////////////////////////////////////////