Add paddle::string::{F,S,}Printf

[wangkuiyi]

For example:

std::string s = string::Sprintf("%2.2f", 12.3456);
string::Printf("%2.2f", 12.3456);
string::Fprintf(std::cerr, "%2.2f", 12.3456);

This implementation depends on a simplifcation and port of tinyformat.

More detailed justification of this PR is in the file comment of paddle/string/printf.h:

// Compared with std::stringstream, there are primary purpose of
// string::Printf:
//
// 1. Type-safe printing, with why and how explained in
//    http://www.drdobbs.com/stringprintf-a-typesafe-printf-family-fo/184401999.
//    Implementation includes
//
//    https://github.com/c42f/tinyformat
//    boost::format
//    std::stringstream
//
//    std::stringstream is not convenient enough in many cases.  For example:
//
//      std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
//
//    boost::format is the most convenient one.  We can have
//
//      std::cout << format("%2% %1%") % 36 % 77;
//
//    or
//
//      format fmter("%2% %1%");
//      fmter % 36; fmter % 77;
//      std::cout << fmter.c_str();
//
//    But the overloading of % might be overkilling and it would be
//    more efficient if it can write to std::cout directly.
//
//    tinyformat has an interface compatible with the C-printf style,
//    and it can writes to a stream or returns a std::string:
//
//      std::cout << tfm::printf(
//                  "%s, %s %d, %.2d:%.2d\n",
//                  weekday, month, day, hour, min);
//
//    or
//
//      tfm::format(std::cout,
//                  "%s, %s %d, %.2d:%.2d\n",
//                  weekday, month, day, hour, min);
//
// 2. High-performance -- most printed strings are not too long and
//    doens't need dynamic memory allocation.  Many StringPrintf
//    implementations doesn't enforce type-safe, but are
//    high-performance, including
//
//    https://developers.google.com/optimization/reference/base/stringprintf/
//    https://github.com/adobe/chromium/blob/master/base/stringprintf.h
//    https://github.com/google/protobuf/blob/master/src/google/protobuf/stubs/stringprintf.h
// 
// According to
// https://github.com/c42f/tinyformat#compile-time-and-code-bloat,
// boost::format runs too slow and results in large executable binary
// files.  So here we port tinyformat.

While I was porting tinyformat.h, I removed support for old Mac OS X versions and some Windows macros. This saved many lines of source code. Let me know if we should add all these back.

[jacquesqiao]

LGTM!

[reyoung]

Amazing library for tinyformat. I will go to details in that library later, but paddle::string::printf.h looks good for me.

[reyoung]

Because this PR is needed by #2646, I just merge it to develop.