Feature - #296 Setting configuration variables in .taskrc

- Now supports 'task config name value', 'task config name ""', and
  'task config name' to directly modify the .taskrc file.
- Updated man page.
- Added unit tests.
- Modified existing config command to also display configuration
  variables that have no values.

src/Config.h

@@ -30,6 +30,7 @@
 #include <map>
 #include <vector>
 #include <string>
+#include "File.h"
 
 class Config : public std::map <std::string, std::string>
 {
@@ -60,6 +61,9 @@ public:
 
   std::string checkForDeprecatedColor ();
 
+public:
+  File original_file;
+
 private:
   static std::string defaults;
 };