Enhancement - history report

- Implemented history report.
- Improved (not to the point of them working) filters.
- Improved automatic filter construction.
- Removed obsolete members from Task.
- Added tag support to the "add" command.

src/Context.h

@@ -72,7 +72,7 @@ public:
   std::string               program;
   std::vector <std::string> args;
   Cmd                       cmd;
-  std::vector <std::string> tagAdditions;
+  std::vector <std::string> tagAdditions;  // TODO This is redundant, remove.
   std::vector <std::string> tagRemovals;
 
 private: