Second try

[spacemoose]

I saw that my previous pull request failed to build for OSX. I was able to reproduce and fix the problem using clang to compile, so I'm trying again.

I put the PrintfArgFormatting changes into one commit, and make the clang error fixes in another. Clang gave undefined symbol errors for the BasicData on the header only tests, so I stopped defining those externally.

I will admit to being unsure of the fixes, so please make any improvements/corrections, or let me know what you need me to change.

• 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. Once we wrap up this issue, I think PrintfArgFormatter should probably come out of internal, but I thought I'd leave that for a next step.

Looking forward to your feedback.

[spacemoose]

Okay, I figured out the windows build issue. Please let me know what you think of the changes.

[vitaut]

I suggest including "fmt/printf.h" for consistency.

[vitaut]

Looks great, thanks for working on this. Just a few minor comments above.

[spacemoose]

Awesome, I'll get the changes done tonight.

[vitaut]

Squashed commits and merged with some changes in 72d51e0. Thanks!