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.

ChangeLog

@@ -20,6 +20,8 @@ Babej).
 - Removed obsolete script 'context'.
 - Fix "project" verbosity info not showing without "footnote" being manually
 enabled.
+- Internal plumbing:  Use variadic templates for format(), reducing code bloat
+and enabling more flexible use of the function.
 
 ------ current release ---------------------------