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/Config.cpp

@@ -36,9 +36,14 @@
 #include "Config.h"
 
 ////////////////////////////////////////////////////////////////////////////////
+// These are default (but overridable) reports.  These entries are necessary
+// because these three reports were converted from hard-coded reports to custom
+// reports, and therefore need these config file entries.  However, users are
+// already used to seeing these three reports, but do not have these entries.
+// The choice was a) make users edit their .taskrc files, b) write a .taskrc
+// upgrade program to make the change, or c) this.
 Config::Config ()
 {
-  // These are default (but overridable) reports.
   (*this)["report.long.columns"]  = "id,project,priority,entry,start,due,age,tags,description";
   (*this)["report.long.sort"]     = "due+,priority-,project+";
   (*this)["report.list.columns"]  = "id,project,priority,due,active,age,description";