Plumbing: Use variadic templates for format()

- Use variadic templates, with a slight twist for properly supporting
  Taskwarrior's l10n strings.
- Adjust code that relied on implicit type conversion which led to
  problems with template instantiation.
- Move some functionality out of format() in order to keep it generic.

src/TDB2.cpp

@@ -58,7 +58,7 @@ TF2::TF2 ()
 TF2::~TF2 ()
 {
   if (_dirty && context.verbose ("debug"))
-    std::cout << format (STRING_TDB2_DIRTY_EXIT, _file)
+    std::cout << format (STRING_TDB2_DIRTY_EXIT, std::string (_file))
               << "\n";
 }