Project layout and build system improvements (#267):

• The code have been moved to the cppformat directory. Including format.h from the top-level directory is deprecated but works via a proxy header which will be removed in the next major version.
• C++ Format CMake targets now have proper interface definitions.
• Installed version of the library now supports the header-only configuration.
• Targets doc, install, and test are now disabled if C++ Format is included as a CMake subproject. They can be enabled by setting FMT_DOC, FMT_INSTALL, and FMT_TEST in the parent project.

Thanks to @niosHD.