format: Add support for numbered arguments

When translating format strings with more than one conversion
specification, it may be necessary to change the order in which
arguments are applied. POSIX covers this use case by allowing
either numbered or unnumbered arguments, it makes sense for us
to support this as well.

https://bugzilla.gnome.org/show_bug.cgi?id=709907
tests: Add imports.format test

https://bugzilla.gnome.org/show_bug.cgi?id=709907