Unit Tests - color.disable color.pri config.obsolete

- Added unit tests to cover automatic colorization by priority.
- Added unit tests to cover automatic disabling of color when !isatty.
- Added unit tests to cover display of unsupported configuration
  variable in the 'version' report.
- Added support the '_forcecolor' configuration variable to allow the
  possibility of unit tests that test color support, yet redirect
  output to a file.  This configuration variable will not be
  documented.

src/task.cpp

@@ -290,7 +290,9 @@ int main (int argc, char** argv)
     if (!isatty (fileno (stdout)))
     {
       conf.set ("curses", "off");
-      conf.set ("color",  "off");
+
+      if (! conf.get (std::string ("_forcecolor"), false))
+        conf.set ("color",  "off");
     }
 
     TDB tdb;