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/Context.cpp

@@ -31,6 +31,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <sys/select.h>
 #include "Context.h"
 #include "Directory.h"
 #include "File.h"
@@ -84,6 +85,26 @@ void Context::initialize (int argc, char** argv)
       args.push_back (argv[i]);
   }
 
+  // Capture any stdin args.
+  struct timeval tv;
+  fd_set fds;
+  tv.tv_sec = 0;
+  tv.tv_usec = 0;
+  FD_ZERO (&fds);
+  FD_SET (STDIN_FILENO, &fds);
+  select (STDIN_FILENO + 1, &fds, NULL, NULL, &tv);
+  if (FD_ISSET (0, &fds))
+  {
+    std::string arg;
+    while (std::cin >> arg)
+    {
+      if (arg == "--")
+        break;
+
+      args.push_back (arg);
+    }
+  }
+
   initialize ();
 
   // Hook system init, plus post-start event occurring at the first possible
@@ -263,6 +284,7 @@ int Context::dispatch (std::string &out)
   else if (cmd.command == "pull")             {      handlePull                  (out); }
   else if (cmd.command == "diagnostics")      {      handleDiagnostics           (out); }
   else if (cmd.command == "count")            { rc = handleCount                 (out); }
+  else if (cmd.command == "ids")              { rc = handleIds                   (out); }
   else if (cmd.command == "_projects")        { rc = handleCompletionProjects    (out); }
   else if (cmd.command == "_tags")            { rc = handleCompletionTags        (out); }
   else if (cmd.command == "_commands")        { rc = handleCompletionCommands    (out); }