Hooks: Implement "Hooks v2" API

- All hook scripts now receive key:value pair command line arguments.
- Add only the basics: api, args, command, rc, data.  Possibly more to
  come.

src/Hooks.cpp

@@ -520,6 +520,30 @@ void Hooks::assertFeedback (const std::vector <std::string>& input) const
   }
 }
 
+////////////////////////////////////////////////////////////////////////////////
+std::vector <std::string>& Hooks::buildHookScriptArgs (std::vector <std::string>& args)
+{
+  Variant v;
+
+  // Hooks API version.
+  args.push_back ("api:2");
+
+  // Command line Taskwarrior was called with.
+  context.dom.get ("context.args", v);
+  args.push_back ("args:" + std::string (v));
+
+  // Command to be executed.
+  args.push_back ("command:" + context.cli.getCommand ());
+
+  // rc file used after applying all overrides.
+  args.push_back ("rc:" + context.rc_file._data);
+
+  // Directory containing *.data files.
+  args.push_back ("data:" + context.data_dir._data);
+
+  return args;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 int Hooks::callHookScript (
   const std::string& script,
@@ -546,6 +570,8 @@ int Hooks::callHookScript (
   std::vector <std::string> args;
   int status;
 
+  buildHookScriptArgs (args);
+
   // Measure time for each hook if running in debug
   if (_debug >= 2)
   {

src/Hooks.h

@@ -57,6 +57,7 @@ private:
   void assertNTasks (const std::vector <std::string>&, unsigned int) const;
   void assertSameTask (const std::vector <std::string>&, const Task&) const;
   void assertFeedback (const std::vector <std::string>&) const;
+  std::vector <std::string>& buildHookScriptArgs (std::vector <std::string>&);
   int callHookScript (const std::string&, const std::vector <std::string>&, std::vector <std::string>&);
 
 private: