Feature #559

- Added feature #559, which implements a new configuration variable,
  rc.exit.on.missing.db, which causes taskwarrior to exit if the
  rc.data.location is missing (thanks to Sander Marechal).

src/commands/CmdShow.cpp

@@ -140,6 +140,7 @@ int CmdShow::execute (std::string& output)
     " echo.command"
     " edit.verbose"
     " editor"
+    " exit.on.missing.db"
     " export.ical.class"
     " expressions"
     " extensions"