Enhancement - #326

- Added feature #326, allowing tasks to be added in the completed state,
  by using the 'log' command in place of 'add' (thanks to Cory Donnelly).
- Added log command to task.1 man page.
- Added log command to task-tutorial.5 man page.
- Added log command to help text.
- Added log command unit tests.

src/main.h

@@ -56,6 +56,7 @@ bool nag (Task&);
 
 // command.cpp
 int handleAdd (std::string &);
+int handleLog (std::string &);
 int handleAppend (std::string &);
 int handlePrepend (std::string &);
 int handleExport (std::string &);