Fix printf filter to allow reordering boost::format positional directive...

That's my approach to handle [boost::format](http://www.boost.org/doc/libs/1_55_0/libs/format/doc/format.html) positional  directives.

The patch basically treats the boost  directives `%1%` like it's printf `%1$` pendant. 
At the same time it prevents treating `%%` (a literal %) as a directive.