Enhancement - Hooks

- Implemented API::callTaskHook.
- Implemented Hook object inside Hooks.cpp, not Hooks.h.
- Implemented Hooks.setTaskId to provide context for task hooks.
- Implemented pre-tag, post-tag, pre-detag, post-detag
  events.
- Implemented pre-file-lock, post-file-lock, pre-file-unlock, post-file-unlock
  events.

src/Task.cpp

@@ -27,6 +27,7 @@
 
 #include <sstream>
 #include <algorithm>
+#include "Context.h"
 #include "Nibbler.h"
 #include "Date.h"
 #include "Duration.h"
@@ -34,6 +35,8 @@
 #include "text.h"
 #include "util.h"
 
+extern Context context;
+
 ////////////////////////////////////////////////////////////////////////////////
 Task::Task ()
 : id (0)