Add a simple "hello fmt" project which tests the compiler flags

[0x8000-0000]

There seem to be an ebb and flow of regressions when building with stricter compilation flags. I suggest adding a simple "hello fmt" project that is built using the following flags for GCC and Clang:

-Wall -Wextra -Werror -pedantic -Wconversion -Wsign-conversion -Wunused -Wshadow -Wdouble-promotion -Wcast-align -Wnull-dereference

With this small project the automated builders could catch the build breaks.

[0x8000-0000]

In our huge code base at ${BIG_CO}, each of these flags would have pinpointed bug locations. Yes, some of them are very noisy. But we're working through getting them cleaned up, one dozen at a time.

[DanielaE]

We do the same in our $(SMALL_CO). We find real bugs by doing so.

[vitaut]

Most of these warnings are already there: https://github.com/fmtlib/fmt/blob/master/CMakeLists.txt#L70 . PRs to enable and at the same time fix additional warnings (within reason, -Weverything is not a goal) are welcome provided that they don't obfuscate the code too much.

[0x8000-0000]

Opened #983 .

We are close to enabling -Wsign-conversion warnings (interestingly enough it is not part of -Wconversion on GCC but it is part of -Wconversion on Clang).

[vitaut]

Fixed by #989, thanks!