Implemented #335 custom printfargformatter

[spacemoose]

As discussed. Summarizing the changes:

• printf.h created and placed PrintfArgFormat ting classes in this file.
• Created a DefaultPrintfArgFormatter, analagous to the ArgFormatter class for the python-style formatter.
• Added another template parameter to PrintfArgFormatter, enabling customisation of the PrintfArgFormatter through subclassing, as with the BasicArgFormatter.
• Created a very simple unit test for the new functionality in custom-formatterer-test.cc

I hope the changes are acceptable. I open to any changes of course. If accepted, I would suggest taking PrintfArgFormatter out of internal in a next issue.